What Are Blue Coloured Cysts On Back Called?

Hi I have always suffered from ingrown hairs. i had a polinital cyst surgically removed years ago. I am a women from a Middle eatern background and have a blue colored cyst on my hamstring region. Ive had this before and with Erythromicin it went away. What are these blue colored nickel sized cysts called? Why do I get them?