documentclass{article}
usepackage{tikz}
usetikzlibrary{fit,arrows,calc,positioning}

begin{document}
tikzstyle{b} = [rectangle, draw, fill=blue!20, node distance=3cm, text
 width=6em, text centered, rounded corners, minimum height=4em, thick]

  tikzstyle{c} = [rectangle, draw, inner sep=0.5cm, dashed]
  tikzstyle{l} = [draw, -latex',thick]

 begin{tikzpicture}[auto]
  node [b] (a) {hello};
  node [b, below=of a] (b) {hello};

 end{tikzpicture}
 end{document}

tikzstyle{X} = [....]

draw [X] (a) --{hello} (b);

documentclass{article}
usepackage{tikz}
usetikzlibrary{fit,arrows,calc,positioning}

begin{document}
tikzstyle{b} = [rectangle, draw, fill=blue!20, node distance=3cm, text
 width=6em, text centered, rounded corners, minimum height=4em, thick]

  tikzstyle{c} = [rectangle, draw, inner sep=0.5cm, dashed]
  tikzstyle{l} = [draw, -latex',thick]

 begin{tikzpicture}%[auto] <- commented this
  node [b] (a) {hello};
  node [b, below=of a] (b) {hello};
  draw [dashed,thick] (a) -- (b) node [midway,fill=white] {hello};

 end{tikzpicture}
 end{document}