I am looking at a pre-ban Colt r6701 flatop 20" h-bar. My question is ; will this gun accept a pre-ban upper16" from Bushmaster, RRA, etc. Or will I have to stay with Colt........Also What does the term "blocked" mean when describing an ar-15?, Thanks

Yes, it will accept any upper but you may need an "offset pin" if the lower is a large hole.  It's really not a big deal, the pins cost around $6.  I have one on my SP1 lower with M4 upper and it's perfect.  I think the large hole dia is .315 and the small hole dia is .250 ... someone please correct me if I'm wrong.
The term blocked means a sear block has been installed, meaning you cannot install an auto sear to make it full auto.  Not a big deal but try to stay away from a receiver that had one installed and then removed.

Hope this helps - Marc

Thanks Marc!