Is it normal for my 57 year old mother's spine mri to have every disc completely black? (every disc)

Yes: As discs age, they lose moisture content (desiccate). This shows up as loss of the normally bright signal on t2 images of an MRI scan -- they get dark. When the discs bulge, or worse, herniate posteriorly into the spinal canal, then problems such as spinal stenosis or nerve root compression can arise. This can cause pain and/or nerve injury in the lumbar spine.

Black disc disease : Black disc disease is a controversial topic when attempting to identify pain generators in the spine. It usually is not an operative problem. On a certain MRI sequence (t2) the disc is black versus a normal white one due to loss if hydration. I see many patients with no neck pain and no symptoms who have black discs and an MRI of the neck for other reasons.