wouldn't a cell deficiency lead to it depleting also the magnesium levels in the blood, simply based on osmosis?

The organism dumps Mg on the blood as much as necessary because it needs to remain in stable concentrations.

When a serum Mg test comes out low, the patient already has severe deficiency and visible symptoms, so isn’t very useful unless you’re investigating a systemic issue.

As explained by my endocrinologist.

If blood concentrations drop, you die.