I think it's generally called "rounding out" (check on google with "flavoring" etc.)
Not sure how to turn that into a noun, but I did find a couple of good refs to "rounding flavor" (UK flavour). I can't guarantee it's the industry standard term - if there is one - but it should be clearly understood.
e.g.
“As a result, we have developed a line of all-natural, vanilla-type flavors, which can be used to accent products such as caramel ice cream or can be used as a rounding flavor in such products as chocolate or coffee ice cream.”
http://www.dairyfoods.com/CDA/ArticleInformation/features/BN...