I've been researching for a bit, and looks like (unsurprisingly) Code::Blocks uses the wxWidgets library.
More specifically, it makes use of the `wxAuiNotebook` widget, part of the wxAui sub-library, for displaying movable/stackable tabs and panels.
Then there's `wxAuiTabArt`, a base class for providing various pluggable back-ends/sub-classes.
Code::Blocks makes use of the `wxAuiDefaultTabArt` back-end for drawing the tabs, which gives that characteristic wxWidgets-y feel to the application... and in the case of ReactOS, a nice `#0000ff` color in the lower part of the disabled tabs surface.
That's where the problematic interfacing code resides. I'll keep investigating.
CodeBlocks main window source code, search for `wxAuiTabArt`:
Here's the culprit, the code that draws the background tab gradient when disabled. I've yet to see from where that coloring comes from:
Here's the Windows implementation of `GradientFillLinear`. Something's fishy over there, I don't like the way they calc the vertices' indexes: