After four years, I finally had occasion to use the blue label cartridge, only to have the tape refuse to feed. The mess on the tongue sticking out shows the result after I forcibly pulled the tape from the cartridge:
The proximate cause was a fold in the imaging tape takeup path causing the driven spool to stop turning:
Some delicate unspooling, unfolding, and respooling put things back in order.
However, with the cartridge opened on the desk, it became obvious this was the cheapnified version:
Compare with a genuine Brother cartridge:
In the genuine cartridge, the base tape (with the sticky side and the colored side) feeds from the lower right directly into the assembly pressure roller. The transparent cover tape feeds from the spool in the lower left, up around the imaging tape supply spool, has the image fused to it, and is then pressed against the base tape on the assembly roller.
Update: Per david’s comment, the cartridges are even more complex than I thought! The printer has sense pins matching a group of cartridge holes to determine (at least) the tape size & orientation. See the pix added below.
Despite using the same cartridge body, the cheapnified tape path is entirely different. The base tape now feeds from the spool in the lower right through what should be the cover tape supply reel, around the imaging film supply spool, has the image fused directly to it, then passes out through the assembly pressure roller.
The cover tape is completely missing!
It turns out the cheapnified cartridges don’t bother with lamination. Instead, the printhead presses the imaging film against the top of the base tape, leaving the black image exposed to the elements. The assembly roller does nothing, apart from pulling the base tape through the cartridge.
Now that I know what to look for, the visible difference is the orientation of the base tape. A cartridge with the correct innards feeds the base tape with the colored side + image facing away from the long side of the cartridge. A cheapnified cartridge has the color + image facing the long side, with the major benefit of making the advertising look more appealing:
A genuine Brother cartridge would print the image on the bottom of the tape in that picture, so you’d see the blank side of the tape in that picture.
The “Amazon Marketplace” being what it is, I assume any pictures will not, in general, have much in common with what you actually receive, but at least I now know which ones to reject out of hand.
Update: The PT-1090 label printer has cartridge sensing pins:
And the cartridges have corresponding holes, although the printer doesn’t sense all of them:
Despite that, cheapnified cartridges are still cheapnified.
I learn something new every day around here! Thanks!
The Smell of Molten Projects in the Morning 
I’m not sure that’s right. How does the printer know to print the image mirrored so it comes out right-reading on the other side of the tape? I think OEM makes cheapnified, non-laminated tapes in addition to the good ones, and there’s a sensor pin to tell the printer which way to print, so it’s not determinative of aftermarket-crap.
You’re absolutely right: I never noticed the sensing pins inside the printer!
I’ve added some pix to the post, so I’ll never make that mistake again …
Some of those pins also convey things like tape width and thermal energy level required. I’ve never looked for the details…