I know this was posted in 2009 but I this is one of the first threads that comes up when searching for how to change a Black and Decker miter saw blade so I figured I would add my answer for anyone else who may be searching and will find this.

I have a Black and Decker Firestorm and to the right, at the very end of the motor housing (the big bulky orange part where your handle and such are), you can insert an Allen wrench or hex key and while it technically does not lock, it provides counter resistance while you are trying to turn the nut at the blade side.