How do I create a mask over a jpeg using the outline within a pattern?

Hello - New to AI so not sure if this is a simple fix or not.

I created a black repeating pattern and filled a large rectangle with the pattern. There is no background colour within the repeating pattern. I would now like to use the pattern to mask over a jpeg that I have placed into the document (the blue portion in the picture attached). Basically I want to have the black portion of the pattern take on the blue colour within the jpeg. When I create the mask it uses the entire rectangle, and not the portion with colour. Any suggestions?

Thanks!