This link was posted a while back in another thread. Very interesting article on jellyfish worldwide.
http://www.nytimes.com/2008/08/03/science/earth/03jellyfish.html
The population increase is global.
The decline in the sea turtle population is also global. All species of sea turtles around the world are declining.
Our oceans are out of balance.
Lady D, what you saw might have been one of the big pink ones. From a distance and when the sun is shining brightly, they do look black. The first big pink one I saw, I first thought it was a ray. Then as I got closer, I thought it was two rays and one of them had been gutted.

Then I got even closer and I realized it was a big pink jelly casting an enormous dark shadow.
I had a nice swim yesterday and saw no jellies at all, one of only a few jelly-free days since spring.