\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\foreach \x  in {1}
\pgfmathparse{100*\x}
\filldraw[fill=blue!\pgfmathresult!white, draw=black] (0,0) rectangle (\x,\x);
\end{tikzpicture}
\end{document}