Black scuzz underneath temporary dental crown, what % hydrogen peroxide to help clean?

You are correct that peroxide should be used to clean this.  Do not do this on your own.  Have your dentist clean this properly for you when bonding on your permanent crown.  Good luck.  

There is no way that you, the patient, can fix this, but when the dentist removes the temporary, you are right to think that Hydrogen Peroxide should be used.  I find that many dentists don't know about this "trick" and spend a lot of time trying to remove this discoloration.In MY office we use a HIGHLY concentrated H2O2 (commonly used for root canal therapy) as it works very quickly and disinfects the tooth very well.  Over the counter forms (is that the 3%?) may work as well, but may not work as fast.  If your dentist does not know about this process, be sure to let them know.