I don't think you can go by the colour of the flowers. I've got 34 different varieties of apple trees and the colour of the blossom varies enormously from a deep pink to a pure white. Ditto timing - one pear flowered way back in February and now has decent sized fruit on it, some of the apples are still coming into flower now. That's why you have to be careful to plant trees that will cross-pollinate because otherwise you could end up with some trees that just don't overlap their blossoming period with anything else.
Anyhow if you think it's a plum, it probably is a plum! The bees will know as well. I was very worried with my early pear because it really was out all on its own except for the very last couple of days and I didn't see a single insect near it but some creature managed to do its job thankfully.
Let us know!