Check it out ... half of the top 10 finalists from 'Idol' -- including Jack Blocker, McKenna Breinholt, Kaibrienne, Mia Matthews and Kayko -- stopped by Nashville's 2nd Annual Franklin Summer Bash to serenade their dedicated fans. However, just as the show came near its end, a storm rolled into town -- resulting in a lightning strike hitting a transformer and causing power to go out at the venue.

The blackout happened in the middle of McKenna's performance of Hillsong's 'Oceans' ... but it didn't prevent her from finishing her set. Watch the vid, after a brief pause, McKenna continues to belt out her song, with the crowd handling the lighting by turning on the flashlights on their phones.

Jack, who was the only performer left to go on, received a similar welcome ... as the crowd kept their lights on for his 40-minute set. Despite the technical difficulties, he persevered ... using just his guitar and his voice to entertain the audience.

The performers -- including 'Idol' Top 14 finalist Quintavious Johnson and "The Voice" star Maddi Jane -- even mingled with fans at the end of the show. In fact, they sat at the edge of the stage in the dark while signing autographs and snapping pics for fans.