Thank you Benjamin Dod, for your answer. I took a while, trying to follow your instructions because I had no idea how to do the things you said to do. After reviewing the Wiki-Manual, I managed to understand and follow your steps 1 to 3.
Unfortunately, the shark acquired some trap doors
So on to your second suggestion to delete the faces at the stripe, in preparation to fill them back in (something I have no clue how to do yet), I suddenly noticed that there are TWO vertices in one place – and thus appeared the answer to my problem!! I selected all the vertices in my shark and removed doubles. Viola, my shark no longer has a blue stripe on its white belly.
So thank you for your aid! I very much appreciated it!