I have been having lightheadedness for a while. Would a CT of head and an MRI with contrast of brain detect blockages in arteries?

Consult with MD. You should consult with your doctor. At your age, there are several causes for lightheadedness. Your blood pressure, heart, and possibly your brain may need to be evaluated. Lalso, that could be caused by medications you may be on. Talk to your doctor.
Might, but has high . Head ct/mr may should arterial blockage. Would be better seen with brain mra.
No. Assuming the other more common causes of lightheadedness such as medication, etc. Have been excluded...A cta (ct angiogram) of the head and neck should show any arterial narrowing/blockage. An mra of the head and neck (no contrast) will similarly show arterial narrowing. Ct head without contrast will only show calcific atherosclerosis and MRI brain with contrast is not used to evaluate arteries.