(The Phoenix) — When the Theater District’s Cure Lounge ejected a group of black Harvard and Yale alums and grad students last month, many saw it as the latest confirmation of Boston’s racist core. It is a perception — as old as Bill Russell and as current as Skip Gates — that successful, professional-class blacks will find more hostility than welcome in the Hub. The Cure incident also caused heartburn for city leaders. With two major black conferences coming to Boston this summer, they hope to prove how far Boston has come in terms of race relations. The Urban League conference in July and Blacks in Government (BIG) the next month are expected to bring a combined 13,000 visitors to the city — breaking what has been, in effect, a quarter-century boycott of Boston. If something like the Cure incident were to happen during those conferences, it could do exactly the opposite, setting the city back years.