Salmo,
The belly loins are white, softer, and have more oil. The dorsal loins are more firm, less oily, and white-slight pinkish hued. It's where the two meet, along the lateral line, that there is a small wedge of meat that contains a lot more blood vessels, and is really dark red. Some call it the 'blood line' but it is just meat, with more blood flow, but tastes very strong. There is maybe a handful of it out of a whole fish, so not much to trim off and throw away.

Here, everyone study this... you should be able to cark and trim a tuna is 90 sec.