mysterious spots

bought for 99p in a charity shop, just needing a small teapot, and thought the blue crackled glaze quite nice.
drank two cups of tea before i noticed the strange dark spots.
you can wipe them off, but they appear each time you fill the teapot.
absolutely no idea what it is, will have to see if my ceramic artist pal knows because its probably something in the glaze.
also probably why it was only 99p!