I have a large bulge in my left neck vein i have seen a cardio doctor all my test have been good doctor wants blood work done now why would he want to?

Many possibilities: Without having the ability to examine you and have your full records it would be speculation. This may be a rare venous aneurysm or a lesion extrinsic to the vein pushing it out. If so, cardiologist may not know how to evaluate and is shooting blindly in hopes of hitting something. Sounds like you might want to see a good pcp.