Unwanted blue line with TikZ

• for sure this image is not a cause for showed blue line
• for fun and exercise i re-code your mwe to

\documentclass{memoir}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                backgrounds,
                calc,
                positioning,
                quotes,
                shadows, shapes}

%-------------------------------- show page layout, only for test
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%
\usepackage{lipsum}

 \begin{document}
 \chapter{First Chapter}
 \section{First Section}
\lipsum[11]
 \begin{figure}[ht]
     \centering
     \begin{tikzpicture}[
node distance = 6mm and 12mm,
            > = latex,
   box/.style = {rectangle, draw, thick,
                 minimum size=16mm, align=center,
                 outer sep=0pt},
 block/.style = {box, draw, thick, minimum size=8mm},
 quant/.style = {box,
                 append after command={
        \pgfextra{\let\LN\tikzlastnode
        \draw[very thick]
        ($(\LN.south west)+(2mm,2mm)$) -| ++ (3mm,3mm) -| ++ (3mm,3mm)
                                       -| ++ (3mm,3mm) -- ++ (3mm,0mm);
                    }% end \pgfextra
                            }% end after command
                },
   sat/.style = {box,
                 append after command={
        \pgfextra{\let\LN\tikzlastnode
        \draw[-Straight Barb]
        ($(\LN.south west)+(1mm,2mm)$) coordinate (a)
                                       edge  ($(\LN.south east)+(-1mm, 2mm)$)
        ($(\LN.south west)+(2mm,1mm)$)  to   ($(\LN.north west)+( 2mm,-1mm)$);
        \draw[very thick]
        ($(\LN.west)+(2mm,0mm)$) -- (\LN.west -| \LN.south)
                                 -- ([xshift=3mm] a -| \LN.south)
                                      node[above right,inner sep=1pt] {f};
                    }% end \pgfextra
                            }% end after command
                },
%every label/.append style = {font=\small}
                    ]
%
         \node (n1) [sat,label={[anchor=north] AAF}]    {};
         \node (n2) [block,right=of n1]                 {S/H};
         \node (n3) [quant,right=of n2,
                     label={[anchor=north]Quantizer}]   {};
         \node (n4) [block,right=of n3]                 {Binary\\ encoder};
%
    \draw[-Stealth] ($(n1.west)-(1,0)$) edge ["$x(t)$"]     (n1)
                    (n1)                edge ["$x_1(t)$"]   (n2)
                    (n2)                edge ["$y(k)$"]     (n3)
                    (n3)                edge ["$y(k)$"]     (n4)
                    (n4)                 to  ["$z$"]        ($(n4.east)+(1,0)$);
    \end{tikzpicture}
     \caption{Generic scheme for an ADC.}
     \label{fig:generic_adc_scheme}
 \end{figure}
 \lipsum[12]
 \end{document}

which gives

• defined symbols in my mwe might be usefull in your other tikz pictures
• from discusion in comments below your question follows, that source of your problem is externalization of tikzpicture
• one among possible solution is not use externalization but draw each image as separated document using standalone package for document class. for example as:

\documentclass[tikz, margin=0pt]{memoir}
\usetikzlibrary{arrows.meta,
                backgrounds,
                calc,
                positioning,
                quotes,
                shadows, shapes}

 \begin{document}
 ... image code ...
 \end{document}

and than obtained pdf file use as image:

 \begin{figure}[htb]
 \includegraphics{<file name>}
 \end{figure}

... just my 2 cents :-)