Unnamed blocks are a glitched block from RedPower 2 obtained by breaking a moving support frame with a block breaker. It will have the item ID 762:1 and will have an invisible texture.

DO NOT pick them up. Don't even touch them.

If you put them in your inventory your client will immediately crash and you will crash every time you try to connect to the server. The only way to fix this is to have the items manually edited out of your inventory, or to have your player file reset (which will delete your inventory). I'd suggest contacting an Admin to dispose of the unnamed blocks safely if you come across them.

Just wanted to warn everyone so they don't make the same mistake I did.