All I did from your sample was set the Grid Background like you did. Before I did that, the transparency on the WinForms PropertyGrid slowed everything way down, and caused flicker by its splitter. After I did it, there was no more flicker and resizing speed performance was dramatically better.
I don't see any artifacting while resizing any more after that one change. If you are referring to a black area that temporarily shows on the far right when you drag the window's right border to expand it, I'm not seeing that but that is something with WPF in general. Basically as a window expands, if the layout hasn't yet "caught up", the black area is the background behind where WPF renders. Then as soon as the WPF layout catches up, it renders over it.
You might want to consider looking at our own WPF PropertyGrid control if you are going to use WPF. It's built natively in WPF and offers many of the same features the WinForms one does.