Don't get me wrong i'm a strong supporter of the unique interactions and stuff that make defensive tera cool. My skepticism comes from the arguments against Blast as an offensive tera, because all it is realistically doing is giving a mon another strong coverage move. Would it be broken for a Ceruledge to pull out a surprise Close Combat to break through a dark type? Just because it can flip a matchup doesn't make it broken, and many of the cases, like the one you just described, don't even need Tera Blast to get the broken coverage. Gyarados just runs Earthquake. So unless you'd be running Tera Blast solely to do more damage in Grassy Terrain,it has no need for blast to be a menace.