Cherries are actually very closely related to plums and the two are similar in a number of ways, including the fact that they both tend to flower early. Plums usually flower earlier than cherries, though that is not always the case.
Both of these plants grow only under warm conditions and those who are thinking about growing them will not want to do so where there are frequently strong cold winds which could damage them.
Both of these plants grow best in heavy limy soil which has to be well-drained in order for them to grow properly and come to full maturity.
Yes, Plums that matted with red cherries long ago produced dark cherries.