You can think of the denominator of a time signature as a whole note split in as many pieces, so that a time signature that is "weird" (the name is actually "irrational", nothing to do with the mathematical sense) like 5/7 now makes sense as "five sevenths of a whole note", or a bit longer than a 2/4 measure.

When notating multiple flat notes in a single measure (e.g. B-flat, or "a semitone lower than B"), the symbol only needs to be applied to the first note in that measure.