Do they?
I would hope that Black Cherry, Raspberry, Cranberry, Cherry, Mixed Fruit, Strawberry and Watermelon don’t all actually have the same flavor… 
It’s been a lot of years since I had Jello. I had no idea there were so many flavors available!
I don’t recall much of a difference between Black Cherry and Cherry, but there certainly was a difference between Cherry and Strawberry. Strawberry reminded me of medicine I often had to take. Yuk! Don’t recall Watermelon back in those days…is that a new-ish flavor?
I think it’s been maybe 25 or 30 years since I last had Jello, and as far as I knew they had like five flavors. Unless they had others that no grocery stores carried, I’ve got to think they’ve expanded the selections in the intervening years.
When I was a kid, I used to take different flavors of Jello and make layered Jello concoctions by enjelloing one flavor, letting it set in some glass custard dishes, then pouring another flavor on top, etc. I think I made a four-layer one, once.
And yes, they taste different. They use the same kind of flavor extracts that are used to flavor hard candies. The flavors are generic but not particularly subtle.
Watermelon, black cherry, and strawberry are all distinctly different IMO.
And raspberry was definitely the best red flavor!
eww… no.
Heck, cherry and black cherry taste differently ( I buy the sugarless versions once in awhile in those six-pack snack versions ).