TM1 system files with the blb extension, incorrectly referenced as "cube formatting" files (admin guide p.35) are actually "rule formatting" files for the "standard" rules editor (prior to 9.1).
The rules editor actually displays the contents of the .blb if there is one, otherwise it defaults to the .rux.
Unfortunately things can go wrong, and the .blb file gets desynchronised from the actual .rux or just go blank.

It is possible to preformat dynamic slices by using the Edit Element Formats button in the subset editor. However that formatting is static and will not apply to new elements of a slowly changing dimension. Also it takes a long time to load/save when you try to apply it to more than a few dozen elements.

As an example, we will demonstrate how to dynamically alternate row colors in a TM1 report for a Customer dimension.