Speaking from experience, sometimes, the best thing a pokémon can do is simply show their trainers that they’re there and that they acknowledge their trainer’s existence. Even simply being in the same room as your trainer can go quite a long way (although physical signs of affection certainly don’t hurt). If you’d like to be more active, sometimes, small things can go a long way too. For example, if you or your teammates are capable of doing small tasks around your home (cleaning or cooking are the foremost things), this may cheer your trainer up a bit—or at least help her relax and let her know that there are others who quite literally care for her.
Another important thing to know is that humans aren’t often as open about what’s on their minds as pokémon are. It may take time before your trainer is ready to talk about what’s troubling her or even to express the exact emotion she’s feeling. Be patient with her and let her express herself in her own way. If she shares with you, all you have to do is listen.
Best of luck, Blossom! May your trainer feel better soon!