Could be something like bot-fly larvae (happened to my dog once.. Really disgusting!)... the larvae is transported by a mosquito and after a bite inserts itself under the skin and develops for 8 weeks unless its pulled or mushed out... but they're usually whitish, so even if you mushed it it should have had some lighter material mixed in... But when they come out, it leaves behind a hole.
My Dog Has A Sore On Him That Has Black Stuff Coming Out. So I Squeezed Him And More Black Stuff Came Out.now He Has A Hole On His Skin. What Can This Be?
It could also be a sebaceous cyst and if so it could keep refilling and coming back. Best to take him to the vet as you don't want that hole getting infected and he will be able to say exactly what it is and give you the right treatment.
It could just be a blackhead that has now stretched the skin pore out. Keep it clean and dry of moisture and it should heal/fill in but if in doubt, see your vet. FOR SURE see the vet right away if it's looking red, raw or infected.