Toka = best choice in my opinion 
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I see this is a Zone 5 thread. I’m trying to create a list of Zone 4 plums for variety assuming either multiple trees or multigrafting and featuring black knot disease resistance. Since I have slightly different goals, should I create a new thread?
Once you deal with colder than zone 5, I believe your plum choices diminish rapidly. Mount Royal is the first to come to mind, maybe the only I know about. It originated in or near Montreal, Canada circa 1903.
I know it is very hardy, handling 16°F (about -9°C) in mid-October on Marianna2624. In this region, many years the first frost hasn’t come yet by that date. I know naught of its black knot resistance (pardon the internal rhyme).
May I recommend using St. Julian A stock? It triggers early dormancy, which saved Ersinger from damage that morning, because it was already devoid of leaves. Mt. Royal on Marianna2624 still had all its foliage; no leaf damage while most apple leaves turned gray.
BTW, each year Ersinger yields fruit - it is partially self-fertile - I am more impressed with the tree and fruit. It would easily offer pollen to Mt. Royal, which might begin a day earlier than Er. Despite Raintree’s estimate that Kirke’s Blue blooms a bit later than Ersinger, their blooms coincided this year, beginning to end.
Kirke hasn’t dealt with an unusual cold snap here, so I cannot speak to that. It also stands on St. Julian A.
I just put Seneca/Marianna2624 in the ground here, several years after its graft. Maybe next year it will bear fruit & I can report.
What about Toka? Isnt Toka supposed to be super cold hearty?
Edited to add whoops! I should’ve read all the comments as Toka was already mentioned.
I tried to find a black ice tree because it looked like a good plum to me. Anyone have opinions on flavor and fruit quality? I’m in zone 8a so not super cold.
You made me look: Minnetonkaorchards.com says Toka blooms with Superior and Shiro, which puts it with Asian/Japanese plum blooms. Very hardy, yes; at least PSF.