No, you will not be able to find Mesprit in Pokémon Black, you would have to trade with Black 2 or White 2 or you would have to transfer Mesprit from a previous game.
You can find mespirt in : Dimond Pearl Platinum HeartGold Soulsilver Black 2 White 2 Pokemon Mysterdungeon Darkness/Time/ Sky (Maybe the new Mystery dungeon) (You may be able to catch mespirt in the new games, Pokemon X and Y)
luck
they are mespirt, azelf, uxie, and dialga
No, they won´t
you use meanlook on him so he cant escape and you catch him
Use the marking map to track her down.
Mespirt is always on the move but if you check your map in your pokewatch thinging on the bottom of the screen there will be a flashing dot and that will be Mespirt or another legendary pokemon! ~ShayminSkyForm~
Giratina from the disortion world, Azlef, Uxie, and Mespirt from their lakes (Mespirt starts roaming after you talk to it) and Dialga and Palkia at Spear Pillar (you need their respective orbs, you find them in a previously inaccesable area in Mt. Cornet) Darkri shamin crestela and hetran
palkia, heatran, giratina, uxie, mespirt, azelf, regigigas, cresselia, phinoe, manaphy, darkrai, shaymin, arceus
To catch Mespirt you have to use the poketch app Pokemon tracker which you get from the poketch president after acquiring the veilstone badge. Using that you can track mespirt, i'd recomend flying to it until it ends up in an easy to track area. Then you just ride your bike or run until it is in the same area as you. Then you walk into the grass or water nearby and it should come up. Note: this only happens after the three legendary Pokemon return to their respective caves. Also, if you have a masterball of course that will make it easy to catch. I'd recommend chasing it with a high level Pokemon that has falseswipe to avoid killing it. I used my level 67 scyther but i had to chase it for awhile. You will only be able to throw one pokeball or attack the mespirt once, so i'd bring a Pokemon that also knows block like nosepass or another escape blocking Pokemon. I hope this helps and good luck.
well the most rarest pokemon i would say is unixe azelf and mespirt they suck i know but there the rarest in the game and dialga and girantina are rare to but not as rare as mespirt, azelf and unixe.
I think you have to beat the eliete Four i know mespirt you catch in the wild use your mark map chart thing