What tool can I use to block light from a 50W bulb?
Photographers use Cinefoil to block light. They attach these foils to the barn doors of lights with pegs. You could try to clamp such foil to the shade, but make sure the peg itself is heat resistance. Don't fully cover/wrap the bulb/shade with foil. Bulbs can get extremly hot after some time. The heat of the bulb should have enough space to exit.

Examples of Cinefoil:


https://www.amazon.co.uk/gp/product/B00915LB6M/ref=oh_aui_search_asin_title?ie=UTF8&amp;psc=1
https://www.bhphotovideo.com/c/product/44690-REG/Rosco_RS100113_Matte_Black_Cinefoil.html
https://static1.squarespace.com/static/50aa7079e4b040d142112688/t/576c2863893fc0c13dd81ad0/1466706022795/ (Cinefoil attached to barn door)


Downside: They are quiet pricey. Therefore a new shade might be cheaper.