I've never heard about it blocking selenium uptake. However, when the body synthesizes MSM, it converts into DMSO, which *in theory* would then make any supplements, vitamins, etc you're feeding along with it be absorbed and utilized better. Definitely an interesting question OP, and one I hope someone with more knowledge chimes in on, as it would cause me to reconsider my current feeding routine.
I learned of the problem with Selenium and MSM at the USDF convention many years ago when it was in Portland. The vet lecturing on sport horse nutrition and care (her name escapes me) made a special point to tell the local attendees about the risk. Sorry, I cannot site the studies, but I believed her!