Like someone already mentioned "rebar in block wall?".
That's new to me, too. I really thought that the OP meant poured concrete with rebar.
Any block wall that I've drilled into there was only one concern and that was hitting conduit or other. Otherwise, draw a circle and use the longest 1/4" bit you can find and drill as many holes as possible. Then knock it out with a hammer.
But if rebar is involved then I can't offer anything since I have yet to encounter block walls with rebar.
