As per your query,you are getting recurrent attacks of swelling with pus.
It is sebaceous cyst
,swelling which occurs due to hair follicle .The swelling has punctum,a dark area over it at the site of hair.
As it is painful,it got infected.Now,you have to get it drained and use antibiotic and anti inflammatory drug.
The dark spot is punctum which will remain after drainage
Next time,when the swelling occurs,get it removed completely with cyst wall and along with punctum.
Then,there will be no dark spots.
Wish you good health and speedy recovery