First, let make sure you are posting on the right board. Did he move to Michigan, or did his relatives end up in Van Buren County?
I found two William Blackwell's who were born on that date in the social security death index, one who died in 1969 and one in 2003. Do you know which is the correct one? Do you have his obit?
I would suggest searching for him in the 1940 census records. I found one William Blackwell born 1934 in New york, living with his grandparents in Virginia. The family was Black. Does that sound at all like him?
When searching the census, I try and put in less information at first. So, I searched on a william Blackwell, born 1932-1934 in New York. I always give a year range for the birth year, as the person's age may appear differently in the census according to which month the census was taken.