Color gradient TikZ environment

Question

1. I like to create a color change in vertical direction of the "pump Symbol" in the following example from black to blue to black again.

2. The symbol itself should NOT be filled with color. ONLY the drawing lines.

My mini Example doesn't work. Any ideas?

Mini Example or MWE

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{fadings} 
\tikzfading[name=fade gradient, top color=black!80,  bottom color=black!80,
middle color=blue!100]
%% Pumpe Symbol
\tikzset{
  pump/.style={
    circle,
    draw,
    thick,
    minimum size=0.5cm,
    path picture={
      \draw [thick] (path picture bounding box.north) -- 
                    (path picture bounding box.east) --
                    (path picture bounding box.south);
      },
  node contents={}
  }
}
%%%
\begin{document}
\begin{tikzpicture}
 \node at (0,0)[
  pump,
  path fading=fade gradient,
  align = left
];
\end{tikzpicture}
\end{document}

Answer 1

IMHO you need neither the shadings nor the fadings library here.

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{calc}
%% Pumpe Symbol
\tikzset{
  pump/.style={
    circle,
    thick,
    minimum size=0.5cm,
    path picture={
      \path[top color=black!80,  bottom color=black!80,
middle color=blue!100,even odd rule] 
([xshift={-0.5pt}]path picture bounding box.north) -- 
                    (path picture bounding box.east) --
                    ([xshift={-0.5pt}]path picture bounding box.south) -- cycle                 
                    ([xshift=0.5pt,yshift={-(1+sqrt(2))*1pt}]path picture bounding box.north) -- 
                    ([xshift={-sqrt(2)*1pt}]path picture bounding box.east) --
                    ([xshift=0.5pt,yshift={(1+sqrt(2))*1pt}]path picture bounding box.south)-- cycle;
      \path[top color=black!80,  bottom color=black!80,
middle color=blue!100,even odd rule] let 
 \p1=($(path picture bounding box.north)-(path picture bounding box.center)$) 
 in (path picture bounding box.center) circle[radius=\y1]
 (path picture bounding box.center) circle[radius=\y1-1pt];                         
      },
  node contents={}
  }
}
%%%
\begin{document}
\begin{tikzpicture}
 \node at (0,0)[
  pump,
  align = left
];
\end{tikzpicture}
\end{document}

Answer 2

Using shadings library instead of fadings, following code might be used.

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{shadings} 
\pgfdeclareverticalshading{rainbow}{100bp}
    {color(0bp)=(black); 
    color(40bp)=(black);
    color(50bp)=(blue);
    color(60bp)=(black); 
    color(100bp)=(black)}


%\tikzfading[name=fade gradient, top color=black!80,  bottom color=black!80,
%middle color=blue!100]
%% Pumpe Symbol
\tikzset{
  pump/.style={
    circle,
    draw,
    thick,
    minimum size=0.5cm,
    path picture={
      \draw [shading=rainbow,thick] (path picture bounding box.north) -- 
                    (path picture bounding box.east) --
                    (path picture bounding box.south);
      },
  node contents={}
  }
}
%%%
\begin{document}
\begin{tikzpicture}
 \node at (0,0)[
  pump,
  %path fading=fade gradient,
  align = left
];
\end{tikzpicture}
\end{document}