If I understand correctly (and tell me if I don't) using any of the modes in any key keeps you in that key. In B Locrian (or G Mixolydian, A Aeolian, etc.) you are still in KEY of C Major -- no sharps or flats.
I can start a bass line using the first 3 notes of the B Locrian scale, and then, if I walk up the scale, my guitarist can join in in E Phrygian. After the first 3 notes, they are identical. The modes aren't about the notes -- any mode in the same key will have the same notes. The notes are about relation. They allow you to throw in some new intervals in the same key by starting from a new root note -- say A instead of C. Using the key of C starting from an A, you have new intervals to work with, in this case, a natural minor scale versus a major scale.
I do NOT hear that, for instance, E Phrygian is NOT in the 'Key of C Major'. Yes, the Key Signatures LOOK the same, but they do not sound the same. E Phrygian firmly has E as the home tone. If you play the examples I posted you will hear what some of the Modes sound like.
For me, when a key
is established, such as C Major, a scalar line built on a note other than C, does
not qualify it as one of the Seven Named Modes. I would refer to it as a numbered-mode-of-that-Major-scale. An example would be (in the key of C Major), a series of notes: F, G, A, B, C, D, E, F - I don't hear this as F Lydian (F is not the home tone), rather I hear it as the fourth-mode-of-C-Major (if it truly needs a label). But seriously, how often would you find ALL seven notes played in series? Well, maybe a college jazzbo solo.
This works the same if a key is firmly established, such as G Mixolydian. A series of notes, such as, C, D, E, F, G, A, B, C are NOT C Major/Ionian within G Mixolydian, nor is a series of notes B, C, D, E, F, G, A, B a B Locrian within G Mixolydian.
By the way, a Natural Minor Scale is NOT the same as the Aeolian Mode.
Often, music composed in a Minor Key will have a raised seventh tone - making it the Leading Tone, also creating a V7 chord. There may also be a raised sixth tone. The chords will function differently than an Aeolian mode. You will NOT find a V7(b9) chord in the Aeolian Mode.
There are also Modes related to the Harmonic Minor Scale that can be useful - especially the Fifth-Mode-of-the-Harmonic-Minor Scale.
Building off this scale: A, B, C, D, E, F, G#, A (A Harmonic Minor)- pedal a low E, add an E Major chord on top, and play this series of notes - E, F, G#, A, B, C, D, E. You have the Fifth-Mode-of-the-Harmonic-Minor Scale. This scale is sometimes called a Phrygian Dominant Scale.