Thanks for the query to H.C.M. Forum.
Main point of concern is , when tech was not able to draw from vein.
2nd reason is bluish color vein . actually the color of veins are blue in white skinned persons.
However dark color of vein ( blood ) may be due to infection , that too very high.
May be due to anemia
, high cholesterol
Consult one more physician and get his opinion as physical examination is very important .
Get in blood for lipid profile
, serum creatinine
Rare chances of blood disorder.