Help with TikZ externalization tutorial in Texmaker

Consider following MWE from p.672 of the pgfmanual:

documentclass{article}
% main document, called main.tex
usepackage{tikz}

usetikzlibrary{external}
tikzexternalize% activate!

begin{document}
begin{tikzpicture}
    node{root}
        child{node{left}}
        child{node{right}
            child{node{child}}
            child{node{child}}
    };
end{tikzpicture}

A simple image is tikz fill (0,0) circle(5pt);.
end{document}

I compile it in Texmaker with the pdflatex -shell-escape option.

Upon a first compilation of this MWE, the .dpth, .log, .md5 and .pdf files are generated for each of the two figures.

Then, I comment tikzexternalize and make the circle blue by writing

A simple image is tikz fill[blue] (0,0) circle(5pt);.

When I compile the document now, I expect that the previously saved external .pdf file containing the black circle will be used, but I see that the circle has indeed become blue in the pdf of the document (and stayed black in the second external pdf, which is normal).

What did I do wrong? Or did I miss something?