In conversation with a singing teacher, she stated that singers generally prefer to sing in flat keys, as it's easier, agreeing with her own teacher's views. No particular reason was given, but I'm totally sceptical. Is there any substance to this?
Well, everybody has a unique voice range, color, etc. Some people believes certain keys is suited them well. For example, I often transpose songs to a different key, although they are perfectly in my range and very comfortable to sing. I try to listen to my sound and decide by ear which is best.
In addition, there is a theoretical debate since the dawn of common period that every key has a different feeling; and flat keys and sharp ones have very different sonorities. I don't completely agree or disagree with that opinion. Since it has some good basis, it is a good assumption your teacher loves the sound of flat keys.
An ideal musician should be able to play in all keys, practice 28 hours a day and feed himself only with music. They don't exist. All players prefer certain keys, say more than four flats to a pianist or guitarist you will get angry looks. Brass players often hate sharps, etc etc.
TL, DR: A historical debatea utopia and a well personal choice, basically art.
I can see where your skepticism is coming from -- a key is just a key and can be in any range. However, I can think of two possible explanations for why the teacher may be more comfortable in flat keys:
She learned the flat keys first. When I started my jazz studies, I did a lot work with the circle of 4ths. With this method, I spent a lot of time on the flat keys working on the particular exercise I was learning (F, Bb, Eb, etc) before I got around the cycle to the sharp keys (E, A, G, etc.). the teacher may have done something similar (not necessarily jazz) where she worked mostly with the flat keys first.
Sometimes people just feel that something is better than the other. She may like to sing in flat keys because, when she does, she knows that she is singing and a flat key and perhaps she just likes that. This may also be related to Synesthesia; the teacher may get certain feelings from singing in flat keys because she has some sort of synesthetic reaction to something about them.
I agree that there's not much substance to it, but keep in mind that it could just be sheer habit. Especially since her teacher used to tell her the same thing she's saying now, she probably just clung onto whatever he said (which can be both good and bad).