My zucchini starts out nice and green. Then when it reaches 8-10 in, it turns yellow on the ends and narrows down. I have it in a well-drained container in potting soil. The only thing I added was some 15-15-15 spray.
Zucchini Goes From Green to Yellow
Certified GKH Gardening Expert
This is blossom end rot.
Here is a link with more information.