Thanks for your help so far. The darker colour is only half a shade darker than the rest of the nail, no dark enough to be blood I wouldn't have thought. But it's pretty much in a straight line not diffuse. Hubby says I'm making a fuss and the Doctor will probably hardly be able to see it. But it's one of those things that looks like nothing in one light and darker in another. I just like to know what things are I suppose.
Go to the practice nurse if you don't want to bother the doc - or go see the pharmacist at your local chemist. If it's straightforward they can sell you something for it - some chemists have a link to a chiropodist, which may be what you really need.