as for the 4 colors the idear was that a singel creature with a reusable effect can handle whole strategies. hence the Avalanche Riders and Gaddock Teeg can shut down so many decks all on thier own that i felt they were defenatly worth including the extra color. and with 9 sources of black mana and 6 ways to fetch them i dont feel casting a BB-creature will be that big a problem. if you feel uncomfatable with the BB try using that golgari legend for removal (forgot her name ^^)
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Street wraith are there to make sure you get you oath/cemetery online when you need it to recast sporefrog agains agro for instance. your kill options are many in this deck depending on matchup. the range from carrionfeeder eating moggwarmarshal and friends for a fatty to moggwarmarshal weeny hords to raidantdragoon suported cryptrat kind of burn to volrath stronghold not runing out of cards with infinite sporefrog or something.