Darken html5 canvas

Darken html5 canvas

I'm trying to use a canvas image as background here:
http://www.cphrecmedia.dk/musikdk/stage/prelisten.php
The code is as below:
window.onload=function(){
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var imageObj = new Image();
stackBoxBlurImage('coverbgblur', 'canvas', 19, false, 2);
}
Beside the layout stuff that needs to be fixed, I need to darken the image
a little bit, so the text will always be visible, also if I use a white
album-cover. Can I somehow in the code above add a line or 2 that will
darken the image? I know I can use CSS3, but its seems unsmart to create
an extra layer of processing instead of doing it right the first time.
I'm quite new with canvas, so every kind of help is hugely appreciated!