My experience has been that SSPX and indult Mass congregations were as racially mixed, often even more so than the Novus Ordo churches. I resent the implication that racial discrimination is practiced in the SSPX.
There aren't many black Catholics to begin with; it would surprise me if there were very many black Catholic families in the SSPX.
But, racial discrimination, if there is any, is the least of the problems with the schismatic sect known as the SSPX.