Myelocytes and myeloblasts are both types of immature white blood cells that develop inside the bone marrow.
Both are related to promyelocytes, another type of immature white blood cell that myeloblasts develop into. Promyelocytes then develop into myelocytes.
Myeloblasts are typically larger than myelocytes. The fluid within myeloblasts will appear blue when doctors view them under a microscope. This is due to the stain that doctors use to aid in visualizing cell structures. Myelocytes are typically smaller and take on a pink color due to staining.