Carrie Underwood Sings With New Kids on the Block in Nashville

New Kids on the Block's [Wednesday night] concert at the Bridgestone Arena ended up being SO Nashville -- because Carrie Underwood showed up to sing 'Happy Birthday' to Danny Wood & Jordan Knight.

There was also a mini American Idol reunion, when Carrie & Paula Abdul ran into each other backstage. [in case you don't know -- Carrie won Season 4 and Paula was one of the judges.]

Only in Nashville, right?