What number of density do you render them too? Most of mine are at 4k.Also I try to use color backgrounds that help blend in even if its a blue background set to almost look like black.(each one is different.) Another thing is you can adjust the gamma and contrast and brightness after render if you select post render on the render image page. Read the post render tutorial on that link I gave you . You can also blur the image in post render to help the fuzz blend in.