I experience the same thing... Starts as a triggered mood but lasts way too long. For me it is like something flips the "blues" switch but my brain is unable to flip it back to off after an appropriate period of time.

But for me when "The Blues" turns into depression, the symptoms differ/change. My depression is marked by extreme prolonged isolation that I can not get out of. If I am forced around people I can not talk to them. The sadness changes quickly to numbness. I am no longer sad, instead I lack the ability to feel any emotion, and along with that comes the extreme anhedonia, and marked frustration that I can no longer enjoy things that I know used to bring me joy.

I've been trying to educate friends by saying "I'm no depressed, I'm suffering from depression, and although the name sounds misleadingly similar, it is NOT the same thing."