Complementary taste to elderberry ice cream?
I would use yogurt. 

First, yogurt dilutes tastes farily well. It has some taste on its own, but it doesn't clash with most aromas you encounter in other dishes (sweet or savory). You can also add it in large amounts without making a dish heavy or changing its character too much. 

Second, yogurt goes well with ice cream. I eat the combination even when I don't want to dilute anything, just a cup of ice cream with fruit and yogurt as a dessert. I know I am somewhat biased towards yogurt, but the fact that many gelaterias offer such cups mean that I'm not the only one who likes it. 

Third, yogurt is slightly sour (or even very sour, depending on the type you eat), and sweetness and sourness are connected in human taste perception. You can mask large amounts of sugar with enough acid, as evidenced in most soft drinks. 

To make it clear, I am talking of pure yogurt here, unsweetened and non-flavored.