A low power view of a focus of neoplastic plasma cells in the marrow.
These malignant plasma cells from a case of multiple myeloma are well-differentiated. It would be hard to distinguish them individually from benign, mature plasma cells.
These plasma cells from another case of multiple myeloma are far more blastic-appearing. It's not hard to imagine how they might be taken for immunoblasts or myeloblasts at first glance.
What you see above are paraffin-embedded tissue sections. Plasma cells, benign or malignant, can also be examined on smears made from aspirated bone marrow: