The package was blacklisted since it depends on libunrar, maybe it's possible to implement and get accepted upstream code to use unar instead when libunrar is not found.

Debian changes this package for another reason (from debian/rules of calibre_0.9.0+dfsg-1):

# Downloads the current upstream release according to
# debian/changelog, and replace the non-free prs500 TTFs with symlinks
# to their free liberation fonts counterparts. This also updates the unpacked
# source tree to the new upstream version.
get-orig-source:
    set -e; \
    V=`dpkg-parsechangelog | sed -rn '/^Version:/ {s/^Version: ([0-9.]+).*$$/\1/; p}'`; \
    mkdir -p debian/orig; cd debian/orig; \
    wget -O - http://status.calibre-ebook.com/dist/src | xz -cd | tar x; \
    D=`pwd`; rm -f calibre*/src/odf/thumbnail.py; \
    cd calibre*/resources/fonts; \
    rm prs500/tt0003m_.ttf; ln -s ../liberation/LiberationSans-Regular.ttf prs500/tt0003m_.ttf; \
    rm prs500/tt0011m_.ttf; ln -s ../liberation/LiberationSerif-Regular.ttf prs500/tt0011m_.ttf; \
    rm prs500/tt0419m_.ttf; ln -s ../liberation/LiberationMono-Regular.ttf prs500/tt0419m_.ttf; \
    cd $$D; tar c . | xz -9 > ../../../calibre_$$V+dfsg.orig.tar.xz; \
    cd ../..; rm -r debian/orig; \
    find -mindepth 1 -maxdepth 1 ! -name '.bzr*' ! -name debian | xargs rm -rf; \
    [ ! -e setup.py ] || tar xzf ../calibre_$$V+dfsg.orig.tar.xz  --strip-components 2