I decided last fall that I'd try this out this fall after harvesting/foraging our blackberries. This, because last year I washed my berry bowls with some soaping dishes, including my lye solution container. I'd put some water in the lye container and had it in the sink. I noticed some berry juice that fell into the lye container turned the prettiest shade of blue. I sealed it and set it aside, and 30 minutes later it was still that same blue (I couldn't let it sit longer because by then it was dinner time!). I have no idea if it will really turn out blue in soap, but this year some blackberries will be sacrificed in the name of soap-science!