The heat was definitely getting to Duffy. Midway through the Welsh songstress’ Friday afternoon set at Lollapalooza, she had to dab her melting makeup with a towel. “I’m not used to this heat, Chicago,” she said. But she didn’t let what she called “Welsh melodrama” get in the way of a groovy performance that seemed to have a cooling effect on the crowd. Dressed in a sexy sailor outfit, Duffy wrapped her soulful, feathery voice around blues-soaked tunes like “Syrup and Honey.” She even dropped in a rocksteady cover of Solomon Burke’s “Cry to Me.” “I don’t normally do covers, but it’s a festival, so fuck it,” she joked. It was an excellent attitude to adopt as a way to kick off a long and rocking weekend.