Did you get this issue sorted?
Hi there. Sorry, I'm based in the UK so have no experience of using block groups. When dealing with geographies that aren't built into MP I usually just use the centroids and forget about the boundaries. If boundaries are essential then MP is no good because it's an unbelievable hassle to read them in over here, though I imagine it would be a lot easier with US data. Sorry I can't be more help.