Prunus serotina subsp. capuli, the capulin cherry (aka P. salicifolia) of Mexico supposedly has fruit up to 3/4" and is sold for consumption locally. One description says “The fruit is round, with red or nearly black smooth tender skin. The pulp is juicy and may be sweet or acid, agreeable but slightly astringent in flavor. It has a single large stone with a bitter kernel. The fruit is of good quality…”
Perhaps those could be used to improve the US version, unless they’re hardy enough to survive here themselves. I’ve searched online before and came up dry with respect to serotina hybrids.
I finally got an answer about Knudsen black cherry juice from them. No response from an email weeks ago but a recent phone call got an answer today. She named about 8 or 9 dark colored sweet cherry cultivars that go into it in varied amounts. All prunus avium - no prunus serotina.
Thats what I expected, its too easy to go that route since there are plenty of seconds from commercial cherry orchards.
Around here there are a fair number of wild prunus avium which seem to survive. I never cooked with any of them but they would probably be pretty good. The fruits are usually small, sweet, and somewhat bitter. These trees were imported from Europe many years ago.