I was digging through a black powder box that was given to me years ago and found a foot long brass air pump and was wonder how it was associated with black powder

Samir, are you sure it is not a pump for lubricant?
These were regular accessories in the days of breech-seating and schuetzen.

A pump can also be used for water then cleaning the barrel.

I also would have thought it would be a pump to get hot water into the barrel.