The cost reduction applies only to generic mana in the total cost of instant and sorcery spells you cast
The main catch is that youll only be able to use black mana to draw cards
10 Lightning Bolt In M:TG, Red has always been the power of direct damage and the best example of that is Lightning Bolt
White can draw cards from lifegain in various ways, but I personally shy away from Well of Lost Dreams and Dawn of Hope these days due to the heavy mana sink required to draw cards, and the inflexibility of when that mana is required
1 Ureni of the Unwritten Ureni of the Unwritten descends onto the battlefield as a 7/7 with flying and trample, Ureni already commands respect, but its ability is what elevates it far above its peers
At a cost of 6, this is no cheap character, but a 7 willpower can help keep him in play once he's out on the board