How do I mellow out the saltiness in black olives?
You can sometimes get olives that aren't in brine, but instead in oil (often with herbs &amp; garlic which might be a good thing or might not). They're less salty but still have some salt as it's used in the processing. 

If you can't get those, or want to use everyday olives you already have, don't just rinse them but also leave them in plenty of fresh water for a few minutes or even longer before draining well. 

There's also quite a bit of variation between brands. I haven't tested how well the flavour correlates to the salt in the nutrition label.