Ravenuts, what detector are you using? I seem to have gotten a lot of black sand experience the past few weeks and might be able to help. But, if you're finding coins, you are already on the right track. Typically if you are not set even close for those conditions everything will be masked out and your machine will be "blind".
Edit: I read back that you are using the BH machines, which are very basic and not very adjustable. Not much you can do with those to compensate for black sand, or for any mineralization in the soil for that matter since you don't have a ground balance setting or even a sensitivity setting that I can find listed in the manuals. Since you are finding coins, I would just keep doing what you have been doing as it's already better than I would have expected under the circumstances.