How do you get rid of dark spots (mold?) in the bathtub cauking ?

It may be in the caulking or behind it. The only sure cure is to use the bleach to reduce the staining and spray the area well with straight vinegar to kill the spores and let it dry. If the stains bother you, you will have to remove the caulking and put on new. Once you have the old off, wash the area with bleach water and let it dry, then spray with vinegar and let it dry thoroughly before you put on new caulking. I hope this helps you Sharon!

https://www.youtube.com/watch?v=e7nS5_21Z-4

Your best bet is to remove the old caulk and apply a silicone caulk which is mold free. It contains a biocide which is guaranteed to not mold if it is cleaned of soap scum regularly. I have used it before and had no problems. You can purchase it online, Walmart, Home Depot, Lowes and Amazon.

One trick I have tried is squirting toilet bowl cleaner on the caulk and letting it sit for 10 minutes and scrubbing clean. It has worked for me. Here is another article that might help - https://structuretech1.com/moldy-shower-caulk/

You might also try soaking an old rag with hydrogen peroxide and placing it on the darkened area. Let sit there overnight. May take more than one application...just add more peroxide to the cloth and let soak again.

Sharon, I think your caulk is stained, and probably there is no way to clean it. Here's caulk that helps prevent mold: https://smile.amazon.com/dp/B0000DI82L/ref=twister_B078GTYFVW?_encoding=UTF8&psc=1

One way that has helped me with stubborn spots is to soak cotton coil from the beauty supply store in bleach and apply it to the cotton strands to the caulk to soak on the area. It works for me when nothing else does. :)