It could be one of several things - a fungal infection, a bruise due to trauma, or in rarer circumstances it could be subungual melanoma. The only way to know is to contact your GP. They may ask you to send them photos of your nail so they can assess if you need a referral to a dermatologist or if it can be treated by the GP.
Thanks , will keep an eye on it, haven't hurt it,just noticed it a few days ago and was curious to what it could be, I take blood thinners and wondered if it was a side affect, but it's only the one nail,will keep you informed and thx again x
