Why "черника", "брусника", "голубика", "костяника", "клубника" and "земляника" all finish on "-ика" or even "-ника"?
The suffix -ик- is fairly common amongst Slavic languages. Wiktionary has a great explanation for its usage in Proto-Slavic and its descendants: https://en.wiktionary.org/wiki/Reconstruction:Proto-Slavic/-ica
Proto-Slavic is a reconstructed language, so keep that in mind.
The main working theory among linguists is that the names of berries satisfy the meaning 4: carrier of a property. As such черника is a berry that is black (черный) while земляника is a berry that grows close to the ground (земля). Голубика is blue (голубой), and I presume that костяника has a large stone inside (I have no idea—I've never eaten it :)).
Here's what happened - you've chosen a subset of berries to prove some hypothesis which actually doesn't hold true if you care to have a bigger set. True,
-ника is very productive when it comes to berries, however here's (incomplete) list of other berries:
- чёрная и красная смородина
Those are, of course, berries in a non-scientific sense (as far as I remember "малина" strictly speaking is not a berry, just like as "земляника" by the way) - but people percept them as such.
Now imagine someone coming up with a question why name of the berries end with "-ка" or "-ха" (or, "-ня", whatever) in Russian.