Question on Grenades

So I was watching a battle report where a squad of plague marines assaulted through difficult terrain, but didn't suffer the I1 penalty. I don't have the actual chaos codex or BRB, but according to this:
http://bloodofkittens.com/blog/2012/10/22/its-like-tactics-plague-marine-review/

Plague marines do not have frag grenades, instead they only have krak and blight. If memory serves me right, I'm pretty sure you need assault grenades to ignore the I1 modifier. I made a comment on the video and people responded that blight grenades count as both assault and defensive grenades. Yet, nowhere did I find supporting evidence to their claim. So my question is do blight grenades count as both or was I originally right?

blight drones are both, assault and defensive grenades. its on the chaos space marine codex page 66

Alright thanks!