They don't block them generally, but their newer consumer modem/router/WAP "appliances" do. If you use your own, you can set whatever DNS you want, but you will have lower data caps and lose some incentive pricing that you can get if you use theirs.

I'd guess if you get business tier service you have more options also, but I've never had that.

I was on ordinary residential service. At the time, using their device cost more money than BYO, and the data caps were identical (or rather, there mostly weren't data caps).