Can you show a sketch that may explain what you are going to do?
Also, which version of Illustrator are you using?
Added a picture to the comment section now 🙂
In case the blue splotch is a path object, just select both the splotch and the "f" and then Object menu > Compound Path > Make.
It didn't work.. Does it have to be on the same layer as the blue spot
Select both objects and use the transparency pallet
Choose Make Mask, and uncheck Clip
Thank you! Will try this tomorrow! 🙂
What kind of object is your splotch (in Illustrator)?
A vector object or just a (scanned) image?
Its multiple vector objects 🙂