Over a year later - and I found a (better) way that doesn't require you to use uncompressed blockly:
I put a custom css BEFORE the blockly_compressed.js
The custom css contains:
.blocklySvg text {
fill:black;
}
That's it - now you can have pastel (ish) colours, e.g.

You do need some colour though otherwise everything fades out.
I am VERY HAPPY to found this - no need to include closure, etc.
Best wishes
Andy