Juice ("sok") used to mean only fruit juice, and nothing else. Vegetables weren't popular, except tomatoes.
Then it gradually became a common term for any beverage that's not alcoholic, not exactly water, not tea, and containing sugar (or other sweetener). IOW, you may be offered juice, you ask them what do they have, and your choice is coke and two other gassed beverages. The trick to confuse the host is to ask "and of which fruit is that the juice".
The standards have fallen drastically. Blueberry juice used to be pure blackberry; nowadays, the capitalism allows anything to be called anything. You may ask for blueberry, and the waitress may say "we got currant, that's the same" - which it, of course, is not. Then you discover that it isn't currant either - it's mostly apple and grape, with some... blackberry. So read the small print. Generally, stay away from juices labeled in English, these tend to cater to wannabe Westerners who prefer anything that's sorta foreign. There's still one old brand with the slogan "we squeeze the fruit" (mi cedimo voće), the full text of which was once "we don't make juices, we squeeze the fruit".
Serbia has really good fruit, and lots of it, so there's no need to fall for the slimy profiteers. They're only slightly cheaper than the real stuff, so go for the best, it's worth it.