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.
As a result, what you see in the rule editor are NOT the rules attached to your cube and it becomes tricky to pinpoint any issue as the rule tracer gets confused too.

A simple fix is to delete the associated .blb file in the TM1 Data folder and reopen the rules in the rule editor. Well it works only until the next time it goes desynchronised or blank.

From 9.1, you can turn on the new rules editor from the tm1s.cfg:
AdvancedRulesEditor = T

Ultimately if you really cannot do without formatting, consider using an editor with highlighting features and copy/paste the rules.