The blue line appears to be under the black letters and there are tiny dots of blue there also like splattered ink
Teller's stamps. Bank tellers will stamp the bands holding the pack together. Sometimes they miss. Not an error at all.
That's true, but that's only because your photo is taken from the back of the bill looking through it. Notice that the letters of the words NOTE IS and DEBTS are backwards ! Therefore the blue ink is actually on top of the black letters.
I used a microscope Im sorry, im having trouble posting today. All photos were taken from the front of the bill except for the one full pic of the obverse. I held the bill face up under a microscopes using two powers
It always does appear that these stamps are under the existing intaglio printing. I think it's more that the blue ink doesn't stick to the black intaglio printing, and therefore looks like it passes underneath.
I watched a video showing how these are made. These two curved lines and the spatters appear to be a beautiful kind of ink, similar in color to the strip. I cant attach a camera to my phone and i dont have a computer, but being a cashier, and having seen many bank stamps, this doesnt appear to be that. Some day when i get time i will take it somewhere and let them look at it. I will save it. If you could only see it with your own eyes, you would see what im talking about.
Like Dave M said, the blue is overpowered by the black printed ink. Likely a teller's stamp off of the band.
The blue ribbon is interwoven into the security paper long before it gets printed. The color being similar is a coincidence.
Steven, you are correct but look again. The ink in question is on the color changing ink well and under the M in AMERICA. It's from a tellers stamp.