@discographer-- Are you sure that they were ripe?
It's true that all wild plums aren't the same. Some are best eaten fresh, while others are preserved. It's also possible that it may be unsuitable to eat due to the variety or the weather conditions.
It's also possible that they're not quite ripe yet. Although wild plums may not become a very dark red, they should be pink or at least slightly red. That's how I know they're ripe. I don't pick them when they're still yellow.
I recommend trying again when the plums have ripened entirely. Don't be disheartened.