A "Bloom Monster" is a new type of Monster, summoned by Bloom Summon, using a Sprout Monster for that. Their card frame has a lavender color, lighter than Fusion Monsters purple. They are stored in the Extra Deck and count towards the 15-card limit. Also, Bloom Monsters have Levels.
Usually, as both Sprout Monsters and Bloom Monsters have conditions, that have to be met for their Summon, they usually have a slightly higher power level than Synchro or Xyz Monsters of the same Level or Rank. However, as they are not limited to to sharing the Level of their Sprout Monster and also only need one Monster for their Summon, they can be Summoned easier in certain situation than aforementioned other Monsters.
Their name comes from the fact that, after fullfilling the condition they have, Sprout Monsters can blossom and become their true forms, their Blooms.