Sometimes, you just have to pick up the phone.

For the one block that was so much lighter, I blocked it off with carpenter's tape and used a paint brush to brush the background with tea. This works! The varible shading in the other muslin background blocks and border work well with this block as a nice design feature. I do not know how it will stand the test of time but for now it is good. (The quilt shop did not return my email question.)

I am assuming that this is a background fabric, and I know nothing about the quilt.
Do all the blocks have to match? Could you see it as a design feature rather than a flaw?

If possible, you could tea dye the muslin all a wee bit to pull it together more.

Just one idea. Even if it is already constructed, you could carefully do this.

Have you contacted the shop you bought the kits from? Perhaps they have some ideas.

I have completed putting together an online block of the month series and the muslin mailed to me in the kits each month for the applique background is not the same shading. Is there anything that can be done at this late stage?