If soaking (gentle bath) didn't remove it I doubt anything else will either. The blue markers are supposed to be water soluble but heat and time can set it, even heat from being stored in a hot place such as an attic or car trunk or even exposed to sunlight beating in through a window. You can try using a bit of rubbing alcohol on it but I suspect the block will have to be replaced. Let us know how you make out and if you find some miracle solution that does take it out.