I don’t believe Tera Blast is a detrimental move to have in the metagame. Committing a tera type, a moveslot, and use of tera during a game in order to gain a new STAB coverage in certain matchups is high-risk high-reward. I wouldn’t go as far as saying that it’s matchup fishing rather than patching up areas of the team that other members cannot cover. Volcarona is the main exception to the rule, though that speaks more to Volcarona than it does to Tera Blast. If you believe Tera Blast is fishing, you could also easily argue that Tera itself is matchup fishing, except it uses less resources. Tera Fire Kingambit baits in Wisp Pult and Volcarona just like Tera Ground Volcarona baits in Glowking and Heatran. I don’t believe singling out Tera Blast would do much for the metagame. If I am allowed to speculate, Regieleki will drop to UU because it loses its one viable non-STAB special move and gets destroyed by every Ground type; Espathra will stay banned because Tera Fairy + Dazzling Gleam will still warp the metagame, even if the possibility of Tera Fighting/Fire is eliminated; Volcarona will stay in OU or face a suspect; and Serperior will fall down to UU, which it nearly is anyways. Removing Tera Blast will likely balance out Volcarona in exchange for limiting creativity in the builder, and I consider that a net negative for the tier.
As for Volcarona, I’m happy it’s finally had time to exist in the metagame after being abruptly quickbanned before, so I can confidently say that it’s a bit too overwhelming and, in my opinion, the main reason Tera Blast is being discussed in the first place. Unlike some Tera Blast users like Tera Fairy Kingambit, it can be very difficult to anticipate Volcarona’s Tera type, mainly because they are all so good in their own distinctive matchups. The two I’ve struggled against and made use of the most are Tera Ground and Tera Dragon. Tera Ground on offensive sets give it insane coverage and help overwhelm would-be checks like Glowking and Heatran. Tera Dragon on defensive sets enable it to tank hits from common threats like Ogerpon Wellspring and still apply great pressure with Fire and Dragon coverage. The problem is, due to the nature of Quiver Dance, you have to scout the set right away, and it’s very difficult to do so prior to Volcarona using Tera. And there are so many viable options for Volcarona that it becomes very difficult. Tera Grass with Giga Drain, Tera Bug with Substitute + Bug Buzz + Swarm, Tera Water, Tera Poison, Tera Ghost, literally anything could work on a certain team and catch the opponent off guard. The risk-to-reward is too high, and it makes Volcarona a problem. I’m not entirely pro-ban quite yet, as I really like its role of checking physical attackers with Flame Body, but a ban might help increase the viability of bulky offense and balance, which I feel neither are doing well in the meta.
As for Volcarona, I’m happy it’s finally had time to exist in the metagame after being abruptly quickbanned before, so I can confidently say that it’s a bit too overwhelming and, in my opinion, the main reason Tera Blast is being discussed in the first place. Unlike some Tera Blast users like Tera Fairy Kingambit, it can be very difficult to anticipate Volcarona’s Tera type, mainly because they are all so good in their own distinctive matchups. The two I’ve struggled against and made use of the most are Tera Ground and Tera Dragon. Tera Ground on offensive sets give it insane coverage and help overwhelm would-be checks like Glowking and Heatran. Tera Dragon on defensive sets enable it to tank hits from common threats like Ogerpon Wellspring and still apply great pressure with Fire and Dragon coverage. The problem is, due to the nature of Quiver Dance, you have to scout the set right away, and it’s very difficult to do so prior to Volcarona using Tera. And there are so many viable options for Volcarona that it becomes very difficult. Tera Grass with Giga Drain, Tera Bug with Substitute + Bug Buzz + Swarm, Tera Water, Tera Poison, Tera Ghost, literally anything could work on a certain team and catch the opponent off guard. The risk-to-reward is too high, and it makes Volcarona a problem. I’m not entirely pro-ban quite yet, as I really like its role of checking physical attackers with Flame Body, but a ban might help increase the viability of bulky offense and balance, which I feel neither are doing well in the meta.