As a preacher he was eminently successful in awakening the people from a state of spiritual torpor, and winning many souls for Christ. In nearly every service there were conversions, and deep manifestations of the presence and power of God. When he preached at Memramcook, "some were deeply affected;" at French village, he left the people in tears, and the truth had a softening power upon the hearts of the people; and when he was leaving them, "weeping was upon every hand," and they pressed him so hard, that he remained another day, when many were deeply affected, and he left them in tears. On the same day and the one following, he was at Hillsborough, when "it was a moving time, many were in great distress, as appeared from their heaving breasts and weeping eyes;" at Tantramar, "many were remarkably happy," and one little girl of seven or eight years of age, "got up on a form, and told in a wonderful manner, what Jesus had done for her soul," and in this journey of eight days he preached eighteen times, and excepting two meetings, he says, "I know not a single occasion in which it was not evident that many who heard the Word were melted into tears, if they did not cry aloud for mercy."
All through his journal, there are evidences that he was a preacher of great power, eminent in the conversion of the people, for the pages abound with references to the services as "a time of power," where "many were in sore distress" as they hung around him, "eager to catch every word," and "weeping was on every hand," as they besought him to remain longer with them. When preaching one evening a young man trembled exceedingly, and cried out in agony of soul, and about bed-time, the preacher heard him praying and crying in the barn. On one of his missionary tours there were so great manifestations of power, that at Horton many cried for mercy, and others rejoiced and shouted aloud; at Cornwallis the arrows of conviction were felt by some "as they had never felt them before, and wept aloud most of the time;" and at Falmouth, "many felt the power of the word," and rejoiced exceedingly.
There were many notable conversions under his preaching. At Petitcodiac a lady whose sons had been converted looked upon him as a deceiver and opposed his work. "She wrung her hands in great distress, and cried 'O that Black! that Black! he has ruined my sons! He has ruined my sons!'" But she too found peace to her soul, after some days of deep conviction. At Horton a lady who had opposed the work of grace, was laid upon a bed of affliction, and she became so greatly agitated that for three weeks she could hardly sleep, but when William Black was praying with her, she burst forth into transports of joy in finding Christ precious to her soul, shouting, "the Lord has delivered me! O I am happy! I am happy!" All through the pages of his journal there abound remarkable accounts of striking conversions, and of people being stricken down by the power of God.
Churches were organized at the places he visited, nearly eighty persons being enrolled during one visit to Hillsborough and Petitcodiac. There wore notable revivals at Windsor, Cornwallis, Granville, Horton, Liverpool and other places. The most difficult part of his extensive field was at Halifax, where wickedness abounded, and the opposition was so great that at one time, when he was on his way to the city, his friends tried to persuade him to delay his visit, as they feared the press gang, but he went boldly forward, and preached with power.
During his labours he was not forgetful of the needs of the coloured people, who flocked to hear him preach, and many of them were soundly converted. In 1784, he preached to about two hundred of them at Birchtown, and during the year upwards of sixty of them found peace with God. Of two hundred members at Shelburne and Birchtown, there were only twenty white people, and at Birchtown alone, there were fourteen classes in a prosperous condition. At Digby in the following year, there were sixty-six coloured people members of our church.
A study of the topics and texts of his sermons shows that he preached the old doctrines, from familiar texts, easy to be grasped by the people, and he laid special emphasis always upon sin, the need of regeneration, and repentance and faith, and as he pressed home these great truths upon the souls of his hearers, there was seldom a service at which conversions did not take place. Like many other faithful ministers, he was often compelled to mourn on account of the backsliding of the people. These were seasons of depression, when he became subject to severe temptation, and mourned the leanness of his own soul. The beginning of every year however, was a time of refreshing, as he regularly and solemnly made the renewal of his covenant with God.
Despite the fact that the whole province of Nova Scotia and part of New Brunswick lay before him as a wide field of enterprise, he yearned after larger conquests, and therefore in 1784, at the earnest and repeated request of Benjamin Chappel, he paid a visit to Prince Edward Island.
He spent about a fortnight there, preaching in Charlottetown and St. Peters, with small tokens of success, and returned mourning the spiritual condition of the people.
After much thought and prayer, he was married on Feb. 17, 1784, to Miss Mary Gay, of Cumberland, an estimable woman, who had been led to Christ about two years previously under his preaching. She was possessed of gifts and grace as her letters testify, and was eminently qualified for the high duties of a minister's wife.
So extensive was the territory and so great the spiritual needs of the people that the young missionary of twenty three years of age, with a burning passion for souls, wrote to John Wesley in 1783, earnestly requesting him to send missionaries to Nova Scotia, who replied that he had hopes of sending assistance a few months later when Conference met. There being no missionaries, however, sent from Great Britain, he naturally looked towards the United States for help, and a few months after his marriage, he started for Baltimore where the Conference was to be held under the superintendence of Dr. Coke. He travelled by way of Boston and preached twice in the city, when under the first sermon one person was converted, and at the second service several were deeply convinced of sin. As he passed through New York he preached in the Methodist Church, and after the services visited a dying woman, whom he found in great distress about her spiritual condition, and he had the great joy of leading her to Christ, as she died next day, shouting, "Glory! Glory be to thy blessed name!" On his journey he preached at every opportunity and always with blessed results, and before the Conference assembled in Baltimore on December 24, 1784, he gave Dr. Coke a detailed account of the state of the work in Nova Scotia, and the Conference appointed Freeborn Garretson, and James O. Cromwell to labor in that field. Both of these ministers hastened at once to that province, but William Black spent some time in the United States preaching here and there, and called for his wife who was visiting her friends in Massachusetts, she having been born in Boston, and with the tedious travel he did not reach Halifax till the end of May. As he was returning homeward, he and his wife spent over three months in Boston, where he had the honor of laying the foundations of Methodism in that city, "the first Methodist preacher who appeared in New England after the visit of Charles Wesley," says Dr. Abel Stevens. He preached in several of the churches, removing from one to another, as the edifice became too small to accommodate the crowds who flocked to hear the young minister from Canada, until the largest church was filled to overflowing with three thousand people. A gracious revival followed this visit, and as there was no Methodist organization, the converts united with other denominations. After a period of thirty years, he preached again in the city in 1822, and many hung around the pulpit, glad to listen to the man who had led them to Christ in 1785. Six years before Jesse Lee preached under the old elm on Boston Common, William Black declared the old doctrines of Methodism, and witnessed many conversions.