Why was the black legend significant/important?

"but let us admit texas, and we shall place the balance of power in the hands of the texans themselves. they, with the southern states, will control the policy and the destiny of this nation; our tariff will then be held at the will of the texan advocates of free trade. are our friends of the north prepared to deliver over this great national policy to the people of texas? " - rep. joshua giddings, ohio, may 21, 1844 this quote points to what major issue in the years leading up to the civil war?