Black growths

I've had small dark growths under my skin. They are not very painful but feel a little sore to the touch. The growth itself is small (like a rather large blackhead) but very hard and completely black in color. Usually there is a part that stick out from the skin that feel sharp or spiney. They usually appear on my back or under  my arms on my ribs. They seem too large to be blackheads and are difficult to remove. I don't get them frequently; however, I'm wondering if these are normal or if I should see my dermatologist about them.