I downloaded Stockfish 8 at 64 bit from stockfishchess.org/download and loaded it into BlitzIn and ran it just fine. Image attached.
When BlitzIn runs an engine, it uses a command to launch a separate program and direct its input and output to a pipe that BlitzIn can read. So the ability to launch a separate program that is 64 bit only depends on the OS the user is running on, not the program trying to start another program. The program like BlitzIn trying to start another program just uses system calls directly to the Operating system. It’s not running the program from within BlitzIn. If you press CTRL - Alt _delete for task manager when stockfish is running you’ll see its a separate process on the list.
To load stockfish 8 , 64 bit, go to Edit menu at top - Settings menu item then in Settings dialog, the engine tab. by the engine selected at top of that tab, click browse and find the stockfish 8 64 or any other UCI engine you downloaded and select it and BlitzIn will use it as your default engine.