Blue bottomed live rock?

[Ben Broyles]

Here's a question that google cant seem to find for me fast enough. Why would the bottom of my liverock be blue? I recently pretty much nuked my tank because I had some type of algae that consumed the whole thing, took over all my corals and seriously could not be stopped. I had no luck with identifying it and the tank wasnt really THAT old so I didnt have much to lose. I bleached my rock, used baking soda, blah blah, soaked it for about a week and a half, scrubbed it clean, soaked again then put it back into the tank. Everything seems to be ok (except a small dino problem that I'm working out) but when I was rearranging the rocks, I pick them from out of the sandbed and the bottoms are blue! Kinda cool looking but not sure if it's something I should be worried about. All the rock above the sand looks normal but any part that I mashed into the sandbed for stability has turned blue. Any thoughts?

[tebstan]

Did you touch it to see if it's the rock itself or possibly a growth? I've seen some pretty cool looking blue and black sponge growing where there's no light.

[Troy Valentine]

Hey Ben,
Give it a good whiff, does it smell like rotten eggs?