Okay, so the one that immediately springs to mind is Apollo and Hyacinthus from Greek mythology. Apollo accidentally kills his lover Hyacinthus, a prince, with a discus—some versions say Zephyrus, the west wind, blew it off course out of jealousy. From Hyacinthus's blood, Apollo causes the hyacinth flower to spring up, its petals supposedly marked with the Greek letters AI AI, a cry of grief. It's a pretty foundational myth for the flower's origin.
What's interesting is that the 'hyacinth' in the myth probably wasn't the modern garden hyacinth we think of. Scholars argue it was likely a larkspur or some kind of iris, or just a generic 'lily-like' flower in the ancient texts. The story got attached to our hyacinths later. Still, the association is locked in now. The tale pops up a lot in Renaissance art and poetry as a symbol of tragic love and rebirth from grief.
There's also a fleeting mention in Ovid's 'Metamorphoses' about Ajax's blood creating a flower (the 'ajax'), but that's a different, though structurally similar, spinoff. The Hyacinthus myth is the big one, and honestly, it's the only one I can recall where the flower is the central, transformative element of the tragedy.