I would imagine that almost everyone who has ever held a tapestry needle and worked needlepoint has experienced the needle turning black and starting to be unpleasant as it goes through the holes of the canvas. I discovered a number of years ago, totally by accident, that using hand made soaps with natural ingredients stops this. It neutralizes the acids in the skin that causes it, - and I haven't had to discard a needle since.
There is also the factor of constant hand washing to keep our hands clean for stitching with fine fibers, and this can be hard on the skin - especially with commercial soaps, which are detergent based. ( I'm thinking I might need to order the "white" bar, as it has no scents and no exfoliating factors and is totally gentle on the skin - it's good for infants, too.)
Recently, my son Sam gave me a bar (generous, since he had bought a whole box) of the beautifully scented Almond Kiss, and I have loved it - these things last a very long time, so are well worth what they cost - in this case, not much. I used it up, so ordered more on Monday - and received it today. (Wednesday) Remarkable mail order service! Anyway, I had put a link on the side bar on this blog under "good stuff" - (you can also click on it here) so do be sure and go visit often. The blog is lots of fun, and I totally enjoy looking at it, as well as buying their good stuff. My mail box smells awfully good, too.