Well, I've been working on variations of how to do this sort of thing and, at least in Inkscape, have ended up a two pronged approach. Take the layer, and both blur it, and break it up by composing with high frequency noise (This is roughly a "mask" operation), then multiply both with the background.
In Inkscape, I assemble all that as a Filter Effect and apply it to the top level layer for all the "ink". It should be possible to replicate the steps in a raster editor though.
A slight displacement map can work well too.