What is one reason a smear might appear excessively blue with red blood cells?

One reason a smear might appear excessively blue with red blood cells is prolonged staining time. When a sample is left in a stain for too long, especially when using dyes like Wright's or Giemsa stain that are often used in cytology, it can lead to overstaining. This overexposure allows the staining components to saturate the cellular material excessively, leading to an overly intense blue color in the smear. This not only affects the appearance of the red blood cells but can also obscure details necessary for accurate interpretation of the smears and diagnoses.

Correctly managing the staining time is crucial for achieving the right balance in color intensity and definition, ensuring that the cells can be properly assessed for morphology and any potential abnormalities.