What exactly is sourness?

I think I can't actually taste sourness. I know I can sense the other basic tastes—sweetness, bitterness, saltiness—and they seem self-evident (umami's weird so I won't get into it). I also thought all my life that I knew what sourness is, but now I'm not so sure.

I know what sour foods are: lemons, tomatoes, berries, vinegar and all that. I know that unripe fruits are usually sour. But how do I describe sourness itself? How do you describe sweetness without listing sweet foods, without mentioning sugar and honey? I guess If I really had to describe it, sour foods tend to have that tangy, tart smell and the mildly bitter taste. But sourness doesn't seem to come as naturally as the other basic tastes. Like, I taste sugar and I know that's sweet, or I taste Chinese herbal medicine and know they're bitter. And if I taste a slice of lemon I'll also know it's sour, but it's more like I know from recognizing that it's lemon I'm tasting, and recalling that lemons are sour. There is not a clear, isolated "sourness experience" for, like there is for sweetness or saltiness. 

If I really think into it, I can always express the taste of sour food in terms of other tastes and smells. Lemon is a combination of sharp, uncomfortable bitterness with the lemon zest smell. I like tomatoes which taste sweet, almost savory but also light, and have their refreshing tart fragrance. But among all the things I've tasted, I could not identify some unique taste that I can confidently call sourness. There's not a thing in the world that I can taste and go "yep that's sour and nothing else." It is like how you can't explain colors to a blind person; the best you can do is describe the other senses and emotions associated with colors. 

Despite my lack of sour-sense, the concept of sourness itself has never appeared jarring or unnatural, though it does feel relatively mysterious or underwhelming. Everyone knows what sourness is, and of course I knew what sourness is right? Or rather I thought I knew it, perhaps by attributing it to the specific experiences of lemon, or tomatoes, or vinegar. I can still enjoy lemons by their zest and tomatoes by their taste, to some extend at least, even though I seem to be missing out on a whole type of experience.

It's just that sometimes I hear others praising the splendid tart taste of a green apple, or see a vibrant "mouth-watering" depiction for some candy, and I just find my own experience...underwhelming. I am, in equal parts, both confused by and englamoured by other's expressed satisfaction. I guess I've always felt this way, but now knowing my own sour-blindness clearer than ever, I admit an additional feeling of jealousy towards other's acute sourness experience; the feeling that I'm missing out. 

Why have I only recently found out about my sour-blindness? It seems like an essential thing that I should just know about. Well, I think I would if I knew growing up that sour-blindness was possible. Like I'd hear my mom talk about how sour the these new grapefruits she got, so I try one and yeah I can sorta tell they taste bitterer and smell  and grassier than usual, and nothing seems wrong. I don't think anything short of a blindfolded sourness taste test could have convinced me of my lack of sour-sense, so how could I know? People talk about sour fruits and sour vinegars all the time.