Ahm, it seems cycle_list=raw"name=colorbrewer-blues" is the correct one I guess, at least there is no error and everything is grey…
Edit: the corresponding line like this in the TeX output (checked with print_tex()), so it’s incorrect since it’s in curly brackets and not in square brackets:
\begin{groupplot}[group style={group size={2 by 1}}, no markers, legend pos={north west}, xlabel={$\delta$ [deg]}, cycle list={name=colorbrewer-blues}]