Trailer brakes

I don't have much experience with trailer brakes and relative maintenance. Today I removed one of my wheels and noticed some kind of oily black powder residue around the wheel. I assume the powdery stuff is brake residue form a brake shoe dragging slightly but don't know if I should be concerned. The wheel turns freely, does not appear to have been hot and the brake works properly.
Should I look deeper into this or is it OK IYO.
Also any thoughts on how many miles one might get out of factory brakes. I have about 6000 on my trailer now.
Thanks for looking and best wishes.