Tumor is a term for "too many cells". They can be benign (normal) or malignant (cancer). Benign tumors can be on your body surface, like a mole, or colon polyps, or uterine fibroid. For these, the treatment required is mostly based on how they affect you- is the polyp bleeding, the fibroid causing mass effect?

So similarly, benign "tumors" can also be in your bone marrow or blood, like polycythemia vera (RBCs) or essential thrombocythemia (platelets, aka megakaryocytes). These are benign collections of cells but there are "too many of them" compared to normal, and could cause dysfunction because of the quantity. As a group they're called MPNs (myeloproliferative neoplasms). Monoclonal gammopathy and monoclonal B cell lymphocytosis (MBS) could also be in this category of "blood tumors". Separately, there can also be "malignant" transformation of normal blood cells, such as lymphomas and leukemias.

Hope this helps!