When you include the bg_img_fit=1 option, the default settings are used.
You can control the background opacity with bg_opacity=X - X between 0 and 1.
You can set the radius of the Gaussian blur applied to the background with bg_blur=X.
To tint the background, use bg_colourise=X with X being the colour code or name.
If you are migrating from Cloudimage APIv6, you could use the legacy blending mode for blurred backgrond colourise (lc prefix): bg_colourise=lc330000 or bg_colourise=lcblack.