I remember when this was first brought up on 710 at least. I can't say that it wasn't anywhere else (like on this forum) for certain but I don't remember it being anywhere else yet. I will try to go back into their archives and find the main "episode" in reference to this but here is how I remember things:
First a couple of the shows had been tossing around nicknames for the Seattle DB's and Bob & Groz decided to do a segment and ask fans to send in some names. They got a bunch of suggestions but the one that seemed to stick was "Temple of Boom". They kind of all agreed on that but it seemed something was kind of missing. Then After an interview with Chancellor later that day (the one where Kam talked about wanting to bring the "Boom"), they continued to read off texts of suggestions and finally near the end of the show one text came in for "Legion of Boom" which Bob read and they Groz really liked.http://mynorthwest.com/category/pod_player_sports/?a=9944700&p=1014&n=
(27 minutes in is where "Legion of Boom" is first mentioned)
Then the next day (August 3rd, 2012) they interviewed Brandon Browner and asked him which he preferred "Temple of Boom" or "Legion of Boom". Browner chose "Legion of Boom" because it sounded "more ferocious".http://mynorthwest.com/category/pod_player_sports/?a=9944746&p=1014&n=
(Legion of Boom chosen at about 55 seconds in)
So unless anyone can find something earlier than August 2nd, 2012 then I think it's only fair to say Bob and Groz coined the nickname "Legion of Boom". If not then they at least had a big part to do with getting it popularized. I for one hope that someone can dig up an old thread on here before then so we can lay claim but I haven't been able to find any yet.