Just a question from any immunologist or cell culture expert who would know what the black granules that look sort of like eosinophilic granules are in the cultured immortalized B-cells.
I am not immunologist but in my opinion the description you provided, it's possible that the black granules in the cultured immortalized B-cells could be a result of cellular debris, contamination, or a reaction to the culture conditions. To accurately identify these granules and their significance, it would be best to consult a specialist or conduct further laboratory analysis.