The following interface collects card backs in a list macro and then processes it once you have a collection of 9 or when you're \AtDocumentEnd:

\documentclass{article}
\usepackage[utf8]{inputenc} %Needed for input
\usepackage{venturis} %CM does not look right at this size
\usepackage[T1]{fontenc} %Needed for output
\usepackage[margin=0.4cm]{geometry} %The layout gets screwed up if I don't include this.
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox (for list processing)
\newlength\cardheight\setlength\cardheight{0.3\textheight}
\newlength\cardwidth\setlength\cardwidth{0.3\textwidth}
\newcounter{itemcnt}
\newcommand{\cardbacks}{}% List of card backs
\setlength{\fboxsep}{20pt}
\makeatletter
% http://tex.stackexchange.com/q/308/5764
\def\ifemptyarg#1{%
\if\relax\detokenize{#1}\relax % H. Oberdiek
\expandafter\@firstoftwo
\else
\expandafter\@secondoftwo
\fi}
\makeatother
\newcommand{\setcardcontent}[2]{{%
\setlength{\fboxsep}{10pt}%
\fbox{\begin{minipage}[t][\cardheight][t]{\dimexpr\cardwidth-2\fboxsep-2\fboxrule\relax}%
\raggedright%
\vspace{0.5 \baselineskip}%
\LARGE #1%
\vfill%
\noindent \normalsize #2%
\end{minipage}}%
\hspace{1pt}%
}}
\newcommand{\card}[3][Cards Against Humanity]{%
\ifemptyarg{#1}{}{\stepcounter{itemcnt}}% Another card has been added
\ifnum\value{itemcnt}>9\relax
\clearpage
\renewcommand*{\do}[1]{\setcardcontent{}{##1}}%
\dolistcsloop{cardbacks}% Set card backs
\renewcommand{\cardbacks}{}% Clear list
\clearpage
\setcounter{itemcnt}{1}% Restart counting cards
\fi%
\ifemptyarg{#1}{}{\listgadd{\cardbacks}{#1}}%
\setcardcontent{#2}{#3}%
}%
\makeatletter%
\newcommand{\cards}[3][Cards Against Humanity]{%
\@for\@c:=#2\do{%
\card[#1]{\@c}{#3}%
}%
}%
\makeatother%
\AtEndDocument{% Flush remainder of card backs
\clearpage
\renewcommand*{\do}[1]{\setcardcontent{}{#1}}%
\dolistcsloop{cardbacks}% Set card backs
}
\newcommand{\blank}{\hrulefill}
\pagestyle{empty}
\begin{document}
\centering
\cards{%Basic set
A big hoopla about nothing,%
A wheelchair death race%
}{Cards Against Humanity: Googling this is probably a bad idea}%
\cards{%The Canadian Expansion
Burning down the White House,%
Terry Fox's prosthetic leg,%
{The Offical Languages Act \\\vspace{\baselineskip}
\noindent La Loi sur les langues officielles.},%To show you some of my cards are a bit more complex
The Front de Libération du Québec,%
Poutine,%
Killing a Moose with your bare hands%
}{Cards Against Humanity:\par The Canadian Expansion. No really, don't Google this at work.}%
\cards{%The Second Expansion
A piñata full of scorpions%
}{Cards Against Humanity:\par The Second Expansion. This if a card game for horrible people.}%
\cards{%The Second Expansion
A piñata full of scorpions%
}{Cards Against Humanity:\par The Second Expansion. This if a card game for horrible people.}%
\cards{%The Canadian Expansion
Burning down the White House,%
Terry Fox's prosthetic leg,%
{The Offical Languages Act \\\vspace{\baselineskip}
\noindent La Loi sur les langues officielles.},%To show you some of my cards are a bit more complex
The Front de Libération du Québec,%
Poutine,%
Killing a Moose with your bare hands%
}{Cards Against Humanity:\par The Canadian Expansion. No really, don't Google this at work.}%
\end{document}
The idea is to use \card as the main card-setting macro, with an optional argument that stipulates what should be printed on the back. Some work may still be required to reorder the backs if they are not supplied in groups of three. However, in your case - with all cards having the same back - this should not be a problem.
List gathering and processing is made possible through etoolbox.
Original solution:
I'd use the following definition for \cardback:
\newcommand\cardback[1]{%
\begingroup\fboxsep=10pt%
\fbox{\begin{minipage}[t][\cardheight][t]{\dimexpr\cardwidth-2\fboxsep-2\fboxrule\relax}%
\raggedright%
%\rlap{\raisebox{\dimexpr-\height}[0pt][0pt]{% Underlay image background
\includegraphics[width=\linewidth,height=\cardheight]{#1}%
%}%
\end{minipage}}%
\hspace{1pt}%
\endgroup}%
It inserts an image to exactly cover the same area as the text. If you wish to overlay it with text as well, uncomment the \rlap{\raisebox...} portion. Then you can add whatever you want since the image will be placed underneath the text. Of course, this requires graphicx. You can extend it the way you want to by means of macros and loops, similar to what you've done for \cards.

\documentclass{article}
\usepackage[utf8]{inputenc} %Needed for input
\usepackage{venturis} %CM does not look right at this size
\usepackage[T1]{fontenc} %Needed for output
\usepackage[margin=0.4cm]{geometry} %The layout gets screwed up if I don't include this.
\usepackage{graphicx}% For images
\newlength\cardheight
\newlength\cardwidth
\setlength\cardheight{0.3\textheight}
\setlength\cardwidth{0.3\textwidth}
\setlength{\fboxsep}{20pt}
\newcommand\card[2]{%
\begingroup\fboxsep=10pt%
\fbox{\begin{minipage}[t][\cardheight][t]{\dimexpr\cardwidth-2\fboxsep-2\fboxrule\relax}%
\raggedright%
\vspace{0.5 \baselineskip}%
\LARGE #1%
\vfill%
\noindent \normalsize #2%
\end{minipage}}%
\hspace{1pt}%
\endgroup}%
\newcommand\cardback[1]{%
\begingroup\fboxsep=10pt%
\fbox{\begin{minipage}[t][\cardheight][t]{\dimexpr\cardwidth-2\fboxsep-2\fboxrule\relax}%
\raggedright%
%\rlap{\raisebox{\dimexpr-\height}[0pt][0pt]{% Underlay image background
\includegraphics[width=\linewidth,height=\cardheight]{#1}%
%}%
\end{minipage}}%
\hspace{1pt}%
\endgroup}%
\makeatletter%
\newcommand{\cards}[2]{%
\@for\@c:=#1\do{%
\card{\@c}{#2}%
}%
}%
\makeatother%
\newcommand{\blank}{\hrulefill}
\pagestyle{empty}
\begin{document}
\centering
\cards{%Basic set
A big hoopla about nothing,%
A wheelchair death race%
}{Cards Against Humanity: Googling this is probably a bad idea}%
\cards{%The Canadian Expansion
Burning down the White House,%
Terry Fox's prosthetic leg,%
{The Offical Languages Act \\\vspace{\baselineskip} \noindent La Loi sur les langues officielles.},%To show you some of my cards are a bit more complex
The Front de Libération du Québec,%
Poutine,%
Killing a Moose with your bare hands%
}{Cards Against Humanity:\\ The Canadian Expansion. No really, don't google this at work.}%
\cards{%The Second Expansion
A piñata full of scorpions%
}%
{Cards Against Humanity:\\The Second Expansion. This if a card game for horrible people.}%
\clearpage
\cardback{example-image-a}%
\cardback{example-image-b}%
\cardback{example-image-c}%
\cardback{example-image-c}%
\cardback{example-image-a}%
\cardback{example-image-b}%
\cardback{example-image-b}%
\cardback{example-image-c}%
\cardback{example-image-a}%
\end{document}