I ended up losing a draft because I had a creature that I could not assign as a blocker, even though it was free to do so. I have the log captured, and the screenshot of it showing up as available to block.

I checked to see if there was some kind of spell cast that forced me not to block - nope - it just wouldn't let me select it as a blocker.

It ended up costing me the draft game, which is really frustrating.