Black status bar icon on Marshmallow making it unreadable on dark wallpaper
Based on reddit (and I have tested it myself), apparently it's a feature of Nova Launcher on Android 6.0 Marshmallow called "Dark icons". It seems it's enabled by default for existing users after updating the app (on my case, new installation will have to turn this setting on manually).

The setting can be checked on Nova Settings - Look &amp; feel - Dark icons. Note that this is a new feature of Marshmallow, allowing an app to have black status icons for light status bar, and thus, the setting is only available on Marshmallow and above.

As for why it also affected Google Now Launcher, I actually have no idea, since I couldn't reproduce the issue on Nexus 5 running Marshmallow 6.0. Even if I leave the option enabled on Nova Launcher, changing the home setting (on Settings - Home - Google Now Launcher) and pressing "Home" button will revert the whole launcher to Google Now Launcher with its default white icon.