Bellossom in depth
The pretty flower and the stinking flower
Its Japanese name, キレイハナ, is a short phrase: kirei (pretty) plus hana (flower). No wordplay, no double meaning. English does the same thing by another route, combining bella with blossom. Both versions say the same thing, and both only make sense in contrast with the other branch, the one that kept the stench and the Poison type.
That contrast is the real joke of the design. Gloom drools and smells bad; with one stone it becomes an even worse rafflesia, and with another it becomes a dancer that sounds like little bells. The Pokédex even links the two things directly: the worse it smelled before, the prettier it will be afterwards. Rafflesia, incidentally, is a parasitic plant in real life that smells of rotting meat, so the original stinker is present in both branches; it just got a skirt here.
What changed along the way
It arrived in Gold and Silver alongside the Sun Stone, which was introduced in that same generation and was a scarce item in those games. Its stats did not move for four generations. In the sixth generation, Defense rose from 85 to 95 and the total went from 480 to 490, a small adjustment that left it with two defenses above 90 and a fairly symmetrical spread.
The other important change is not in the numbers but in the move list. A second-generation Bellossom was a Grass type with Petal Dance, Sleep Powder and not much else. The Scarlet/Violet version has Quiver Dance, Moonblast, Moonlight, Petal Blizzard and Grassy Terrain available from level 1 upon evolving, plus Solar Blade and Leaf Storm by TM. It is the same creature with thirty years of tools piled on top.
Using it in a normal playthrough
Outside competitive play, the problem is not its Speed but when you get it. Gloom appears early and the stone comes late, so many playthroughs reach the end without ever having to make a decision. Once you do, what you gain is staying power: 100 Special Defense absorbs special attacks that would floor other Grass types, and Sleep Powder plus Giga Drain handle long fights without spending items.
Against an ordinary team, its most comfortable role is a second-line Pokémon that switches into a neutral special attack, puts something to sleep and heals back up. What you should not ask it to do is win slugging matches: with 90 Special Attack and no sun, most opponents at its level hit harder and sooner.
A note for anyone playing with the Pokédex open beside them. It is one of the few cases where evolution changes the creature's schedule: Gloom is nocturnal and this one is diurnal, closing its petals at dusk and going to sleep. The stone does not just remove the Poison type; it changes its routine.