Level 100 Statistics (see level 5, 50, 100)
Unfortunately for Bellossom, she's as average a Grass-type as can exist. Thanks to her average stats, her average ability, her average movepool, and even her average typing, Bellossom has practically nothing over her competition. Don't be fooled, though, as this doesn't make Bellossom a bad Pokemon. Her stats are placed perfectly to allow Bellossom to abuse her ability, Chlorophyll, and her movepool has all the right things in all the right places to ensure that she won't be a disappointment if you use her correctly.
This set is about as good as it gets for Bellossom, and is the most useful role that she can fulfill in UU. The main idea is to come in on a Pokemon that poses little threat to Bellossom, such as Milotic or Rotom, set up Sunny Day, cripple an opponent with Sleep Powder, and switch out to a teammate that can abuse the remaining six turns of weather. Bellossom works great early in the game if Sunny Day hasn't been revealed yet and some of the opponent's team has been scouted. The last two slots give Bellossom some auxiliary support to ensure that she isn't completely hapless against the enemy. Hidden Power Fire gets a pseudo-STAB boost under the sun, and pairs well with the Grass-type attack in the last slot for neutral coverage. SolarBeam is the primary choice in the last slot as its synergy with Sunny Day is pretty much a given.
Moonlight can be used in place of Hidden Power Fire to give Bellossom much more survivability and allow her to set up Sunny Day more times during the battle. As a bonus, Moonlight heals 2/3 of Bellossom's health under the sun. Leaf Storm can be used in place of SolarBeam to give Bellossom a one-time power shot at the enemy, but with sun up, SolarBeam is far more reliable. Hidden Power Rock can be used in place of Hidden Power Fire to damage any Fire-types looking to take advantage of the sun Bellossom provides, and it handily eliminates Moltres, a common Bellossom switch-in.
Team Options & Additional Comments >>>
Bellossom doesn't have many other options, as her movepool is extremely shallow. Bellossom can use a pure offensive Sunny Day with SolarBeam, Hidden Power Fire, and Moonlight, but she is easily walled and lacks other coverage moves that her Chlorophyll brethren possess. She has Stun Spore, Teeter Dance, and Sleep Powder, which are useful on double status and Parafusion sets; however, her lack of offense and Speed is an issue. Bellossom can make use of a Swords Dance set utilizing Leaf Blade, Return, and Drain Punch, but the same dilemma occurs as the offensive Sunny Day set. She has access to the SubSeed combination, but she is too slow and can only use it while sun is in play, which isn't reliable. Finally, equipping Bellossom with Choice Specs can give her a lot more power, but her coverage moves end at Leaf Storm, Sludge Bomb, and Hidden Power Fire, so use another Pokemon instead.
Checks and Counters
RestTalk Spiritomb is an awesome counter to Bellossom, as it takes little damage from even a full-powered Leaf Storm, absorbs Sleep Powder, doesn't care about paralysis, and threatens Bellossom with Pursuit if she attempts to switch out. Opposing Fire-types terrify Bellossom by taking advantage of the sun she sets up and threatening to OHKO her with any Fire-type attack. Altaria and Chansey are almost perfect counters to Bellossom because they are not harmed in the slightest by Leaf Storm or SolarBeam, and can heal themselves of whatever status Bellossom may inflict on them thanks to their ability, Natural Cure. Furthermore, Altaria and Chansey can threaten the rest of Bellossom's teammates by either setting up or using status attacks, respectively.