Excessive noise in dark backgrounds is often unavoidable, Brian. I get a lot of problems with macro shots.
Method 1 is to use a little bit of auto noise suppression. I'm not sure what is available with your software though. And you have to be careful because it can soften your main subject as well as blurring the background. But worth a trial. Like this example.
Method 2. Which is what I normally do, is to lightly run over the problem areas with a light Blur Brush (say 20% opacity) and selectively apply more blur where required. Like this edit.
But I am using a drawing tablet and pen. Might be worth you trying with a mouse, going as steadily as you can manage, and see how you get on with it. Possibly try with two hands on the mouse?