I'm trying to put a bathroom in my basement and am redoing a floor drain line. I don't know what or why it would be blocked.

Your question isn't clear to me.
Are you installing a floor drain downstream from a toilet and you're concerned it might overflow? If so you don't need a check valve simply install a Flood Guard, (see image)in the floor drain. They may be purchased at; Flood guards for floor drains - check valve to prevent flooding from FAMOUS PLUMBING SUPPLY (http://www.plumbingsupply.com/floodguard.html)
If it's something else click on back and tell me. Good luck, Tom