What does it mean when the American flag has blue stripes instead of red?

That's a "blue line" flag that supports police.

Which sounds just fine, except it supports police including in what's clearly overstepping their authority, using needless violence, racial profiling, and other police tactics that are often directly against department policy.

In essence, it's often the equivalent of a Black Lives Don't Matter flag. Usually I see it in close proximity to Confederate flags and Trump flags or signs.

Apparently people are hoping to start a fad and cash in - lots of variations on the approved red, white and blue.  A couple:  a flag with one blue stripe:  the "Thin Blue Line" honoring law enforcement . . . . . a flag with blue and black stripes has meanings with members of the Leather community.  I haven't seen blue and white stripes - will keep looking!