Tera Blast prioritizes Special Attack.
Firstly, Bulbapedia states:
Tera Blast deals damage. If the user's Attack stat is higher than its Special Attack stat, Tera Blast becomes a physical move when Terastallized; otherwise, it remains a special move.
Source.
This implies that the Special Attack stat is prioritized, meaning that if the stats are equal, Tera Blast will be Special when the user is terastalized. However, just to be sure, I tested it in game. I leveled up a Quaxly, got its attack stats equal, and went against some Eiscue. Testing both the regular Tera Type and Stellar, Tera Blast was always special (I could tell because Tera Blast damaged Eiscue directly). However, when I modified Quaxly's stats by using an X Attack in battle or by raising its attack stat by one point, Tera Blast became physical (I could tell because Ice Face activated).
Hope this helps!