I had a cervical spine and lumbar spine mri, which showed diffuse mildly diminished marrow signal. neurologist says it could be normal as my blood work is normal. serious?

The MRI finding have to be correlated with your symptoms, physical signs and other laboratory test results in order to make a differential diagnosis. If your neurologist told you that MRI finding is insignificant, you should trust him (her). In any case, you can always seek for a second opinion. Because of limited info you can give us in this forum it is difficult to give you a correct assessment.

Your neurologist is likely very correct, but if you have misgivings about the opinion, suggest you discuss results with a radiologist or seek a second neurology opinion.