THanks for the help guys. Unfortunately, I"ve tried all those thing. Opting out, opting in. deleting blob files. Everything.
EDIT: other steam games work fine. The only "completing installation" dialog I've ever got was in BL2. Before it would just shoot up to 100%. Now it hangs at 1% and then tells me Steam servers are too busy. I've tried during off peak ours, changing servers. The works. All other Steam games work fine. Now I haven't tried all 400 games, but the half dozen I have tried worked just fine
If you want help then ask man. I offered but since you must have missed it I will post my solution
-DL Steam from the site and do a repair install. Obviously have Steam closed/exited during this process. If it halts, ctr-alt-del the process after a few minutes. It basically finished installing but for some reason just hung without letting me confirm it.
-Verify the cache of BL2
-This is the most important step so pay attention
Go to the Borderlands 2 installation folder (mines in C:\Program files\Steam\Steamapps\common\Borderlands 2\)
Run these installer/exe files
...\Steamapps\common\Borderlands 2\Binaries\Redist\DXRedistCutdown\DXSETUP.exe
...\Steamapps\common\Borderlands 2\Binaries\Redist\vcredist_2005_x86.exe
...\Steamapps\common\Borderlands 2\Binaries\Redist\vcredist_2005_atl_x86.exe
...\Steamapps\common\Borderlands 2\Binaries\Redist\vcredist_x86.exe.exe
...\Steamapps\common\Borderlands 2\Binaries\Redist\dotNetFx40_Client_x86_x64.exe
-Reboot your PC and run the game again