What key signature would you write for blues in F?
I personally prefer F Major key signature (1 flat) for the same reason I prefer D Minor key signature (1 flat) for "So What" and "Impressions." I prefer a key signature that shows me at a glance what the tonal center is. I personally find it misleading to think of C Major as the tonal center of "So What," or of Bb Major as the tonal center of an F Blues.
I agree with you on Blues changes; I don't think it's really the same thing though. When you see a I chord in blues, more than 7 notes are suggested, so the key signature primarily tells you where home is and then you have to know the rules of blues to understand the rest. There really is no way around written accidentals with the blues, because it has it's own set of rules regarding both melody and harmony that do not fit within the diatonic confines of a key signature.
With D Dorian 7 diatonic notes are specified. IMHO accidentals should be used to indicate none diatonic tones rather than diatonic tones. With modal music the key signature does not tell you the tonic, it tells you which notes are diatonic. The chord symbol tells you where home is.
I think of modes two ways. The first way is their relative relationship with the Major keys. You can also relate them to the natural minor keys.
The second way is to group them by shape similarity to Major or Minor forms. Lydian and Mixolydian vary from the Major form by one accidental. Dorian and Phrygian vary from natural minor by one accidental. Locrian varies from natural minor by two accidentals.
When I relate modes to key signatures, I use the the first way of thinking about them (how they relate to the Major or Natural Minor key). So to my way of thinking, it makes perfect sense to use the relative Major or Minor key signature for a mode. In a sense Ionian and Aeolian use the same key signature, so why would not Dorian, Phrygian, Lydian, Mixolydian, and Locrian?
In the event someone writes a piece in D minor but wants it to be played D Dorian, they should put a note to say D Dorian. If I were given a lead sheet to sight read, I would not necessarily have time to read through and analyze the melody to determine the song is actually in Dorian mode. If I see a D minor key signature and a D minor chord I will play D Aeolian.
To be honest, if I am not paying close attention I might play D Aeolian even if a D minor chord is indicated over a C Major key signature. But I would consider that an embarrassing mistake if I did, and I would not expect a more competent player to do this.