Background blend mode is not supported if you use 2 different divs.
I see you want to use a hove that will blend in on the image that is not coded in the same div.
you can use blend mode like this:
.project .thumb {
background-image: url(test.png);
background-color: red;
background-blend-mode: multiply;
}
thats the way to do it not in a hover.
If you would like a hover that will do the blend you need to do it this way.
So make a hover with the same image and a blend in color or image.
So the hover will overlay that iamge with the same image but than with a blended color.
.project .thumb:hover {
background-image: url(test.png);
background-color: red;
background-blend-mode: multiply;
}