I have a single-entry Schengen visa from the Belgium embassy. I will not go to Belgium. However I am going to Serbia. Will I be able to use this visa to enter?
Yes; according to both the link phoog posted and Timatic, a valid Schengen visa suffices to enter Serbia.
With that said... if you're coming in with a single-entry visa for a country you don't intend to visit, expect to get some serious questioning on entry. It's the sort of thing that really looks like visa shopping, regardless of whether it actually is. If you had actually intended to visit Belgium and circumstances beyond your control intervened, some documentation of all that would be good to have on hand. And if you have time to apply for a Serbian visa, doing so would save you a lot of worry and potential grief.