I think doing one's own blue cheese sauce should be reasonably easy -- I don't think you'd even need to cut it, because you're adding it to milk and roux which would tone down the flavour. But yeah, if you did need to, a really mild white would probably work. Mozzarella or mild white cheddar -- I'd go with mozz, it's a bit moister.

I've totally spoiled myself for mac and cheese by learning how to cook it myself. I can't make it from the box anymore. It's just so much better home-made!