Actually (...byl), in Ukrainian и is y and і is i (and the pronunciation does differ, unlike in Czech.
So the plant in Czech černobýl, not černobíl, it does not come from the colour of white. It comes from bylina=билина= a plant/grass.
"It is Чорно́биль = Čornobyl', with hard Y, not soft I. Ukranian white has soft I, бі́лий (Czech bílý).
It has rather something to do with Czech bylina = herb., or rather with the plural or collective býlí (not bílý) or Ukrainian билля́ =weed, plants, grass blades, stalks... from Proto-Slavic *bylьje. Ultimately derived from verb *byti (Czech být).
Note that Eastern-Slavic languages, like Ukrainian still distinguish Y and I in pronunciation. Also note that Russian and Ukrainian orthography for I and Y differs.
It simply means a black plant or herb, specifically a mugwort. In Czech černobýl. Not sure if it is a borrowing or common heritage. Many plant names were borrowed in the 19th century."
The stems of Artemisia vulgaris are indeed quite dark
No, not "black and white", the "-биль" part ("-býl" in Czech) means "herb". So "Чорнобиль" (Czech: Černobyl) literally means "black herb", which is also the name of common mugwort, as you pointed out. Not sure why sicne there's nothing black about the herb, at least visually.