The most important tool when deciding cell type is patient picture. What makes more sense for the patient? If they have an increased lymphocyte count due to a viral infection, it would make sense to see some activated lymphs. Blasts however, are not typically seen in response to infection and usually indicate a myeloproliferative or myelodysplastic disorder. Blasts may be accompanied by drastically increased white count or other immature white blood cells. If a suspected blast is seen, it is a good idea to compare it to the other cells in the smear to confirm.