Sure looks like a ball to me. Here's what I'd do...plug the nipple with a tooth pick, stand the rifle up on its butt, squirt a bunch of WD 40 down the bore and let it sit for a day. Now put a ball screw on a range rod, and run it down into the ball. You'll know instantly if it's lead...the screw will bite into the lead. When you've got it in good, pull the ball out. You may need a friend to hold the rifle, or put into a padded vise...it'll take two hands and some determination to remove the ball. Then clean the rifle well. I concur with smylee grouch..use water that is comfortable to your skin, definitely not hot water. Shoot only real Holy Black Powder - not alternative fake powder no matter what the sales people tell you. Welcome back and enjoy the sport.
I see Daryl has beaten me to it...oh well, like minds...