django blocktrans and i18n in templates

I have an i18n problem in django:

This works fine :

{% trans cat.name %}  

cat.name will be translated

But this doesn't work:

{% blocktrans with cat.name|slugify as cat_slug %}{{ cat_slug }}{% endblocktrans %}  

cat.name is not translated

If I change the filter :

{% blocktrans with cat.name|capfirst as cat_slug %}{{ cat_slug }}{% endblocktrans %}  

I can see that the filter is working, but there is no translation...