Obsidian, I've tried them on *top* of the soap, and they do stay blue there, unlike anything else for me except calendula, even on top everything else goes brown over time. I have not used them *in* the body of the soap b/c I tried so many other things that went yucky. Again, except for calendula. I'd be interested to know if anyone has used them *in* the soap too.
I did try google imaging at one point and could not find any that convinced me that they would stay blue if fully immersed in batter, but no discussions on it, unfortunately.