That depends on what you mean by "explore." Unions don't tend to explore issues in the abstract, but rather in relation to specific conditions their members encounter or around strategic objectives they adopt. There are many unions that have conducted anti-racist struggles in hiring, promotion, training, etc. There are probably none that have taken up the issue theoretically or as an abstract principle (although nearly all have some broad convention resolution that asserts a principle). In some respects, the UE has conducted both specific struggle around issues of racism in the electrical industry, while also conducting a more general educational campaign in the pages of its paper and other publications. I imagine other unions could also be cited. (District 65 pre-UAW had a very robust educational life, including programs around racism; 1199 did also.) Coming to terms with immigrant organizing has forced some unions to address xenophobia and prejudice among its U.S. members toward workers from across our Southern borders, Southeast Asia, China, India, Pakistan, Japan, Korea, etc.
Michael E.