I'm not sure depression is at all connected to what we call the blues. Everyone gets sad. Depression is irrational. It can grow out of sadness, but I think this only happens to those who were prone to it anyway. Psychologists have long believed you can develop depression by suppressing grief or trauma, so there's that old nurture vs. nature thing, and I have no answers to it. But depression is very deep, it starts to cause us to not do things or make bad decisions, it affects our essential self-esteem. Sadness isn't like that. Treatment might, however, be useful even for deep sadness that sticks around for a long time just to help snap us out of it, and by that, I mean therapy, not drugs. Drugs should always be confined to those old enough so that their brain is developed if possible and when therapy doesn't work or life gets so affected it's just not working, it's severely disrupted, in which case we're not likely to do well in therapy. It's a great question to raise, Mom, because so many come on here who are grieving and are being given drugs for depression. The problem with that is that it can suppress the grieving process, leaving it unfinished, and that can turn what might have been a bad bout of sadness into a permanent case of depression.
What about dysthymia? That's low level depression which is very similar to the blues. And we can get the blues out of nowhere. Just a 'feeling' in the air.