Dear Paul - thanks for your question and sorry in the delay of my response.  I was not able to find specific information that answered the question about a blood test detecting skin cancer.  You shoudl ask your doctor if he or she knows of something like this.

In searching the site, reading our material and expert and member posts, it seems skin cancer is initially detected by visually seeing a growth of some kind on the skin through self examination.  To determine if it is cancer, a doctor must examine and if necessary complete a skin biopsy on the area.  If diagnosed, a doctor can help with an appropriate treatment plan.

Hope this helps and all the best, sue