> the trials and blocks appear with a black background
As they should with /screencolor = (0,0,0).
> black text background - white font
As it should with /txbgcolor = (0,0,0) and /txcolor = (255,255,255)
> How do I get rid of the extra white dialog box?
Identify what that element is in your /bgstim, /stimulusframes or /stimulustimes attributes and then either remove it (if you don't need it) or set its color to what you want / need.