By DCist Contributor Salima Appiah-Kubi After slogging through a sweltering, sweat-drenched day there is certain contentment that comes from a warm Washington night. This good humor was evident Saturday night at the Black Cat, where a sizeable crowd came out for cheerful indie kids, Tilly and the Wall. The Omaha based group was joined by their label mate, David Dondero, and locals, Exit Clov. The near overwhelming dominance of X marked hands showed the youth…