DR Jennifer - As DR Ron Pulliam noted atlready, do not bleach your light blue shirt unless you want a white spot on your light blue shirt unless you want a white/lighter spot where your stain used to be. Now, maybe you could take a Q-Tip and make some sort of design with the bleach - this way it would look like the "light show" at the top of the second act. Or not.

Yes, bleach is safe for 100% cotton clothes, but only in the sense that the bleach will not break down the fabric itself, the cotton fibers. Some colors are color-fast and may not fade when bleach is applied, but you never know until you try. *And you could "try" on an inside seam to test for colorfastness. However...
There are also those various "Oxy" products you could try.
If you really want the stain out, take it to a dry cleaner/laundromat and have an "expert" look at it. Unfortunately, depending on the type of stain it is and how long it has been set in by now, it just may not come out at all. So, just bring it in, and ask, "Can you get this stain out?", and see what they say.
But the cost of the stain removal may be equivalent to the cost of another
Mamma Mia! t-shirt.
And, remember, it is just a t-shirt.