Let's suppose there are two treatment levels, and so the effect of treatment (your B1) also has two levels. If Black has two levels, then the effect of TREATMENT*BLACK (your B3) has 2*2 = 4 levels. So what does it mean to compare B1 with 2 levels with B3 which has four levels (via the subtraction B1 – B3) ? I suggest that the question as posed is not answerable.
... to test whether the treatment had a non-zero impact for a certain racial group.
Perhaps you are really interested in testing the effect of B3 to see if it is statistically different than zero? If that's it, then B3 can be tested to see if it is zero via the default output from PROC MIXED. You can also use the SLICE command that might provide additional information. ESTIMATE, CONTRAST and LSMESTIMATE seem not needed, in my opinion.
I have the same objection to B1 + B3 that I had to B1 – B3.
I still think the SLICE command may be the tool you are looking for.
Are Treatment and Black continuous (or dummy) variables or are they in the CLASS statement? Can you send us your complete PROC MIXED code and the Class Level Information table from PROC MIXED output?
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