Access to color by color series name does not work

[schtandard]

(I reported this bug via email a couple years ago, but it seems maintenance has changed, so I report it again here.)

The following MWE produces a black square even though it should produce a blue one according to the documentation.

\documentclass{article}

\usepackage{xcolor}

\begin{document}

\definecolorseries{foo}{rgb}{last}{blue}{red}
\resetcolorseries{foo}

\textcolor{foo}{\rule{1ex}{1ex}}

\end{document}

From what I understood, this is due to the fact that the second argument of \xcolor@ in \\color@foo is empty.

As a workaround, one can use the (undocumented) syntax \color{foo!!}, but the documented \color{foo} would be preferred.