A blade of grass would probably be ett grässtrå. You could also refer to the blade itself as en bladskiva. But for trees, both blad and löv work fine. Which one is preferred varies a bit with context. We'd say lövträd 'leafy tree' and höstlöv 'autumn leaves', but I'd probably prefer blad if I'd say something like trädet har gröna blad 'the tree has green leaves'.