Yes, there is a black out material. It's called radio speaker grille cloth.
C and G Parts sells it. Part # 40-18806
https://cgfordparts.com/wwwsectionfi...o_speaker.html When I bought a piece from Drake back in the early 1970's it had the same part # as C and G does today. The color was black. Make sure C and G has the black color as they use the same part # as DRAKE did but it may be a different color as it isn't specifically listed for a 1940 Ford. If you are not building a concourse car you can use black window screening material.
Go to Ebay and look for "Radio speaker cloth material". Most vendors will sell you a smaller piece for your speaker. When it's installed you can't really see the pattern.
Ebay has this speaker cloth which looks like mine.
https://www.ebay.com/itm/Antique-Rad...YAAOSw-0xYcSyj
this also:
https://www.ebay.com/itm/Antique-Rad...8AAOSwttVaIqmT
Folks who restore 40 Fords to OEM specs probably know exactly which cloth is correct.