i might be a fluid that is filled with pus. also i may be infected the screw may be rusty.you need to go to the doctor before it get worse
Something similar happened to my daughter.... turned out there was a sliver of something in that blister. When it finally popped, she had a thorn - in your case it could be a sliver of metal or dirt or oil - Your finger could be infected. Try soaking it in warm salty water and use an antibiotic cream and a bandaid on it. If it gets worse, see a doctor.