No 100% complete burn is somewhat normal. Also, Blackhorn likes when the powder is compacted / compressed tightly. I really lean on the ramrod when loading the bullet using Blackhorn powder. I have a ball starter at the end of my ramrod. A T-handle I use sometimes too.

Lastly, with Blackhorn powder, you should use a hotter primer. Buy the CCI 209M primers instead. Or consider switching to either Federal 209A or Winchester Shotgun Primers, not the Winchester 777 primers.