Hi there
I'm coming in a bit late, but if you are going to make this again, try reducing the amount of vinegar as well as cooking at varoma temp, speed 2-3 with steamer basket over the top. I find when converting chutney recipes, cooking at varoma normally works a treat to reduce the mixture enough, but sometimes I do need to reduce the vinegar as well. If there is added water I never put that in either.
Blood plums are just amazing aren't they? Sooooooo delicious and such a beautiful colour.