some my live rock is from the gulf of mexico , I also have some black spots on some rock
I was told it could have been from the oil spills in the past looks like a tar type substance ,
edit :
looking again at the top pict it looks like 2 mushrooms sitting next to a red sponge
but hard to tell under the blues .
but I do know some rock has black tar looking spots most likely rock out of the gulf from the oil spills
some of mine has black spots been in my tank over 5 years no ill effects