You have quite a bit of leeway to shoot further back on a black bear. The lungs go back quite a way. Do not crowd the shoulder knuckle, and keep the shots reasonably close. They are not very deep top to bottom, so errors in elevation can cost you.
I double lunged a nice boar with a shot that looked to be too far back. He went 60 yards maybe and was dead in seconds. Another was quartering towards me(from 6 yards), arrow went in behind the shoulder and out behind the last rib........he made it a whole 25 yards. I also lost one that I hit in the shoulder knuckle from trying to put it tight in the pocket. Helped a friend track one that was hit too high on an uphill shot............never found it, I think he got one lung.
In my opinion, the best shot on a blackie is quartering away slightly, put the arrow a few inches back of the shoulder crease and not quite halfway up the chest. Broadside is good too.......just be wary of that tight low shot too near the shoulder knuckle. If you feel the need to crowd the pocket, keep it halfway up.
My .02 Have fun stalking bears.
Darcy