A bit late to this conversation but here’s my 2¢ worth.
There are far too many
blue icons out there! Please, don’t make it blue. Far too safe a choice. I’ve always liked the colour as, although it can be a little dark in some contexts, it does makes Sigil easily recognizable.
Talk about how it looks in a drag & drop context in the OS X dock makes me a little wary. If it’s a choice between an icon that looks good in many contexts and one that looks great on a Mac I’d choose the former. Not that it
shouldn’t look good on a Mac, just don’t get carried away in one direction.
I’m not using the standard icon in Ubuntu myself. I’m using an icon theme called Vibrancy Colors, but I believe their Sigil icon comes from Sam Hewitt’s
Moka icon set. I couldn’t attach the source SVG but you can see it
here. (Creative Commons Attribution-ShareAlike licence)
The Launcher (Dock equivalent) is transparent and its colour changes with the wallpaper so visibility under varying conditions is a plus.
Moka icons dark/light vs. Standard (?) icons dark/light
This icon is probably too specific (flat theme, rounded square) to be generally acceptable, but it’s an example of a simple reworking. The idea of a white border would be a simple fix for the problem with dark backgrounds.