Unlike that of earlier works on slavery, Blassingame’s innovative stud

"that" when used with "of" creates a possessive pronoun that needs to refer to some characterstic of the earlier studies, rather than the studies themselves. Additionally, it would be very rare to use "that of" when referring to a plural noun - we would usually use "those of". "that of" would only be correct if we were referring to some communal characterstic that the studies had as a group.

An example of proper usage of "that of" would be:

"Unlike that of earlier studies, the genre of Blassingame's study is..."

More likely, we'd see something like:

"Unlike those of earlier studies, the table of contents of Blassingame's study...".

In the posted sentence, we're comparing the studies themselves to Blassingame's study, so "that of" is 100% incorrect.

As an aside, we can definitely compare a group to a specific person.

The following are proper comparisons:

"Unlike most other hockey players, Wayne Gretzky was more excited about getting an assist than a goal."

"Like her co-workers, Melissa enjoyed a good drink at happy hour."