Hi welcome to Healthcaremagic
Thick blood means increased viscosity of the blood, increased viscosity are more prone to thrombosis
of blood in the heart, calf veins, DVT, stroke
are the possibilities... hence the viscosity must be to normal,,... normally blood wont be black colored... deoxygenated blood will be bit dark compared to arterial red blood.. consult the cardiologist, they will guide you...
Hope I have answered your question..