James Parnell answered The "black, cheesy stuff" that you describe is probably pus which is being excreted from the cyst. The fact that the pus is black makes me believe that the cyst is what is known as a sebaceous cyst. The best piece of advice I can give you is to not squeeze the cyst! Instead, try to keep it clean by wiping it with disinfectant.My dog's cyst burst. What do I do? Sebaceous cysts are usually nothing to worry about, and occur in every breed of dog. They are usually resolved in one of three ways:They burst naturally. They get walled off.They go down by themselves. As your dog's cyst has erupted, this means it will go away shortly - as long as your keep the area clean. Squeezing the pus out of the cyst can cause it to implode, which can then lead to bacterial infections - so is not advisable. As long as you keep the wound clean, you will minimize the risk of infection.