The arrow can be moved by a canvas transformation:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc,fit,arrows}
\usepackage{graphicx}
\begin{document}
\noindent\resizebox{\textwidth}{!}{%
\begin{tikzpicture}[auto,node distance=5cm,>=stealth']
\tikzset{
block/.style= {draw, rectangle, minimum height=2em,minimum width=4em},
sum/.style = {draw, circle, node distance=2cm},
input/.style = {coordinate},
output/.style = {coordinate}}
\node [block] (A) {};
\node [block, right of=A] (B) {};
\begin{scope}[transform canvas={yshift=.7em}]
\draw [->] (A) -- node {} (B);
\end{scope}
\end{tikzpicture}%
}
\end{document}

The length of the arrow (or lines) can be modified by shorten < and shorten >:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc,fit,arrows}
\usepackage{graphicx}
\begin{document}
\noindent\resizebox{\textwidth}{!}{%
\begin{tikzpicture}[auto,node distance=5cm,>=stealth']
\tikzset{
block/.style= {draw, rectangle, minimum height=2em,minimum width=4em},
sum/.style = {draw, circle, node distance=2cm},
input/.style = {coordinate},
output/.style = {coordinate}}
\node [block] (A) {};
\node [block, right of=A] (B) {};
\begin{scope}[transform canvas={yshift=.7em}]
\draw [->, shorten <=5mm, shorten >=5mm] (A) -- node {} (B);
\end{scope}
\end{tikzpicture}%
}
\end{document}

Remark:
- Inside
\resizebox TeX is in horizontal mode. Therefore spaces matter and
the unwanted spaced by line ends need to be removed, e.g. by adding the comment character at the end of line.
[yshift=3pt]? – Symbol 1 Sep 15 '14 at 02:53calclibrary is better if you are already using canvas transformations somewhere else in the picture. – Claudio Fiandrino Sep 15 '14 at 05:10