Is an electrical blank (or anything you have to screw off) an acceptable access panel?
Yes, it is acceptable; you just need access. However, you do need to make sure that the thing that it covers is to code as well.

For example, if you have a standard box, you can splice inside of it and put a blank cover on it. You can't, however, just have a splice behind drywall with a hole that is covered by a blank plate.