I recently bought a 24" iMac (intel based) in April 2009.
Within 2 days of using it, I had a small 3"x3" "ink blob" show up on the screen, very similar to what you're experiencing, but much darker and prominent.
As far as I'm concerned, it's a defective LCD screen and your problem will just get worse. The reason I'm saying it will probably get worse is because mine did and this "ink blob" has grown in size over the past few weeks to a size of about 8" in length and about 5-6" in width. Covers a good fraction on my screen now when it appears.
Meanwhile, it's only appeared 3 times while I've been using the computer. I tend to keep it on when not in use, but don't monitor it's activity 24/7, so it could be happening more frequently than I think.
I contacted Apple support to document the "sitings" (I feel like I'm some paranormal psychic) and they've recommended me to return the product from where I got it (BestBuy - Southern Ontario doesn't have mac stores unfortunately).
Anyways, I'm planning on returning the iMac tomorrow in hopes that a new one will be the fix of this issue, but thought I'd put this out there to anyone else who might have this problem or know of someone who is currently encountering it and going "wth!"