The natives were restless on Sunday afternoon at Lollapalooza, and who could blame them? They had already spent two days on the sun dancing, drinking and rocking out — clearly, their reserves were tapped. Salvation came in a stellar set from indie heartthrobs Black Kids, who took their U2-goes-to-the-disco tunes and let them bounce adroitly across Grant Park. The shimmering chords of “Hit the Heartbrakes” and “I’ve Underestimated My Charm (Again)” let the voices of frontman Reggie Youngblood and keyboardists Dawn Watley and Ali Youngblood dart to the surface with ’60s girl group harmonies and call-and-response dance chants. Sometimes buzz kills a band dead, but the Black Kids have the chops to groove well past the usual expiration date.