Mr. watson's belief that blacks are lazy is an example of his refusal to rent an apartment to a black family is an example of

Stereotype, confirmation bias

Explanation:

A stereotype is a widely held and oversimplified image or idea of a particular type of person or thing. The belief that all Blacks are lazy is a common stereotype which is also a completely false one.

A confirmation bias is the tendency to interpret new evidence as confirmation of one's existing beliefs or theories.

Mr. Watson heard so many times that Blacks are lazy that he can not think otherwise or form his proper opinion. He uses the stereotype and the confirmation bias to judge any black person he encounters regardless of their character or abilities.

sounds good to me mate!