Jay-Z kicked off his "Reasonable Doubt" 30th anniversary concert series with a snip-tease -- with Beyoncé giving his hair a trim in the stands of Yankee Stadium!

Check out the clip -- the buzz was shown before the rapper took the stage Friday night ... and fans went wild as they watched the superstar tidy up her man's look. Fans online are seeing the buzzed look as a sign that new music is on the way, BTW!

But that wasn't the only surprise of the night -- Beyoncé ended up joining her hubby to fill in for Mary J. Blige's feature in "Can’t Knock the Hustle." The power couple had fans thrilled ... and she gushed over the rapper before departing, encouraging the crowd to "Give it up for my baby."

Jay-Z also invited their oldest up on stage, 14-year-old Blue Ivy, to perform the 1996 hit "Feelin’ It" together ... and she played a piano solo at the end! They finished their collab with a sweet hug.

As we told you, Friday's performance was the first of three scheduled for the Grammy winner at Yankee Stadium. Saturday's show will be dedicated to his 2001 album "The Blueprint," and a third "EXTRA INNINGS" concert on Sunday will combine both shows due to high demand.

We'll stay tuned to see if Jay-Z has any more surprises to pull!