I wrote something that does work, but it's extremely ugly, but thought it could either help someone else or we could get something better. Here's what I came up with:
Modernizr.addTest('canvasblending', function() {
if ( ! Modernizr.canvas ) return false;
var canvas = document.createElement('canvas');
canvas.width = 1;
canvas.height = 1;
var ctx = canvas.getContext('2d');
ctx.fillStyle = '#000';
ctx.fillRect(0,0,1,1);
ctx.globalCompositeOperation = 'multiply';
ctx.fillStyle = '#fff';
ctx.fillRect(0,0,1,1);
return ctx.getImageData(0,0,1,1).data[0] === 0;
});