That sounds like the plum
That sounds like the plum tree we had in our yard in So Cal. It was a Satsuma plum. Reddish green on the outside and dark red to purple on the inside. I know it needs a pollinator. We had a Santa Rosa plum with it. The Satsuma was a meaty plum and the skin was not bitter.The Santa Rosa was very juicy and sweet, but the skin was tart. Hope this helps.