The verb condiō, condīre can mean 'to season or flavour (food, wine, etc.)' (Oxford Latin dictionary) or 'to make savory, to season, spice' (Lewis & Short). Although incondītus, the negative adjective formed from the perfect passive participle, isn't listed in OLD or L&S*, it's easy enough to derive it, on the model of indomitus (< domō), infectus (< faciō), and many others.
Meaning-wise, being derived from the perfect passive participle, this adjective should suggest that no extra flavoring has been added to the food's natural flavor, whatever that natural flavor may be like. The other most obvious possibility is insipidus. However, being the negation of sapidus, 'tasty, savory' (OLD), this means 'tasteless, insipid' (L&S). It's focused more on the quality of the flavor itself, especially presenting that flavor as bland and unappetizing. In other words, it's flavorless rather than unflavored.
Therefore, I don't think insipidus is a neutral word in the same way that the English word 'unflavored' is when it's applied to something like yogurt. 'Regular' (plain, unflavored) yogurt isn't necessarily bland; it just doesn't have extra flavoring added on top of its natural flavor. That is, it's incondītus, not insipidus.
* As opposed to incondĭtus (< condō).