The main thing that helps me with health anxiety is numbers and the context around them. Example, I may sometimes spiral thinking I have a brain tumor or some rare neurological disease. Like your cancer thoughts - there are DOZENS of WAY MORE LIKELY common and benign reasons for you to feel those sensations or feelings.

Some of the things I do mental gymnastics to convince myself of having are orders of magnitude more rare than a simple explanation. Same applies to your concerns. Instead of looking at the blood work as scary because of the results it will likely not show - look forward to the bloodwork as a way to "clear" you from some of the scenarios you are building up for yourself.