That's an easy few lines of code:
In ulx\lua\ulx\modules\cl\motdmenu.lua:
Find this piece of code at line 16:
window:MakePopup()
Then add these lines beneath it:
window:SetBackgroundBlur( true )
window:SetDrawOnTop( true )
window:DoModal()
:SetBackgroundBlur is the only thing you really need for the blur effect, the other lines of code just make it so it's always on top, and that you can't switch to any other VGUI/Derma menus.