If you look at C minor, it has a Bb in it but you can't use it. To play chromatically, you need a B natural. Going to F minor, you can't use the Eb to play chromatically but you DO need that Bb. Bb minor has an Ab that it can't use for chromatic play but it does need the Eb.

If we look at some sharp keys, F# minor has a natural E but needs an E# for chromatic play but it doesn't have one, it only has F#, G# and C#. But C# minor has a natural A when it needs an A# (it's only sharps are F, C, G and D) but it does require a natural E.

I find this quite surprising. Our minor scale key signatures are simply wrong.