Two ways I can think of.
use blueberry juice as a replacement liquid instead of water/milk.
dehydrate whole blueberries and grind them, and use them as a portion of your flour measure.
both options at once?
I don’t know if either of those would suffice, and would probably (in the case of blueberries) make for interestingly-colored cake, but I bet it would taste nicely.
As for your plums, you want the SCENT of the plums, or the taste? For the taste, don’t you just juice them and then reduce the juice over heat until it’s all thick and intense? Reduce a bag’s worth of juiced plums to about a half-cup of liquid, and I bet that would be hella strong. Add a crapton of powdered sugar to create the glaze, and you’re set. Now, it might take you a day or more to reduce that much liquid out, and you might not even be able to reduce it that much, but that’s the idea, isn’t it?
For your pans, you need to buy better quality pans. There’s no way to keep the thin ones from twisting that I know of.