That looks like a great place to try windshield urethane adhesive. It is a single component moisture cure adhesive that is very tough, flexible, waterproof, and black in color.
I've used the 3M brand, and a few other brands for replacing windshields. A few years ago, I used some leftover urethane to repair a tear in a pair of GI Mickey Mouse boots, and the repair has held up very well. Anything left doesn't last long in the tube after it is opened, because the humidity in the air causes it to cure fairly quick. You might be able to buy a partial tube from a local windshield repair shop, since you won't need a whole caulking tube, and most will go to waste once it is opened. They might also have a tube that is past the expiration date,. It is absolutely not recommended to use expired windshield urethane when replacing a windshield for safety reasons. But it will still work great for other repairs for some time after the expiration date. I wish they packaged it in smaller tubes because it is really great stuff.