There is no single blood test to rule out all cancers . Basic routine cancer screening is not required at your age until unless you have any unusual symptom .
If you are really worried and want to do specific screening for any symptom please consult , and would be able to suggest you screening test for specific cancer .
CEA test , CA 125 , CA 19.9 test , AFB , LDH , combined with routine blood examination and X rays are few of the test that could be of some use in your age . Again reminding no need of routine cancer screening at your age unless you are having any symptom.
And the black spot you are seeing on your body (neck) are not cancer they are skin tags they usually don’t require treatment,if causing pain or for cosmetic reasons can be treated with small interventions .
Next Steps
Please do consult me , before doing blood test , since knowing your symptoms and complaints fully , I may be able to suggest you more septic tests.