My mom has chronic bone pain in back, arm, chest, shoulders, neck, ribs, knees, hands. blood test = calcium deficiency. worried about it being cancer metastasis. but if she had cancer that metastasized, surely it would have shown up in blood test?

Cancer may, may not (probably not) show up in “blood tests.” If low calcium, may be due to endocrine (Parathyroid) problem or malnutrition. Needs to see an Endocrinologist.

Imaging is usually needed to rule out metastatic cancer as no blood test can rule it out. It does sound that a systemic disease is causing the generalized pain, her treating doc remains your best reference.

Blood tests only pick a minority of tumors. It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some imagining tests. It would be prudent to see a doctor. Wish you and your mom good health!