Unwanted newlines in LaTeX export

Consider the following custom block:

```
#+begin_blockquote :citekey key :pages pages
*This* is a /try/.
#+end_blockquote
```
which was defined like this

```
(org-defblock blockquote nil (citekey "" pages "") "This is just an attempt."
(format (if (equal backend 'latex)
"\\blockcquote[%s]{%s}{%s}" nil)
pages citekey contents))
```

What it should print:

```
\blockcquote[pages]{key}{\textbf{This} is a \emph{try}.}
```

What it does print:

```
\blockcquote[pages]{key}{
\textbf{This} is a \emph{try}.

}
```
The problem is that, due to the added whitespace, the custom block just shown produces the latter output. If this is exported to LaTeX, instead of `"**This** is a _try_"`, you get `" **This** is a _try_ "`. regexp-replacement does not work here, adding `%` only for the first line. Any ideas what to do?