I was getting a PO446 code I changed the vent on the cross memeber under the truck few days later the code came back. I didnt mess with it for a while do to the fact the motor in my daughters car blew. SO I solved her problem and now its back to the blazer. I noticed the other day when the gas cap was off that the little tab inside the filler next is stuck all the way open. Could this be causing my code?

 

Nope

Read this... P0446 code - GM-Trucks.com
{GM TSM's dont always get it right}

Is there any chance you flooded the canister? Otherwise, I'd go for checking/cleaning purge solenoid and valve.