Command is classified a Blocking spell because it can be used in reaction to stop an incoming attack. Only Blocking spells can do that (Regular, Missile, Area and Enchantment don't have that as part of their description), so there's not much of a choice for the category.
Yes, in the game context this requires appropriate wording (like "wait!" or "freeze!") and situation (like an up-close melee attack, as opposed to a sniper 3 km away who can't hear you). As usual for GURPS, the rules are not meant to be proof against lawyering by munchkins, attempting to meticulously enumerate every detail of every situation of a spell's possible employment, but are assumed to be employed to play a game by cooperating adults that are using common sense, using a description that's generally only a few sentences long.
The Blocking category doesn't mean it's the only way Command can be cast, nor is a defense the only possible use for Command. (Being one possible use, or even the most important use, doesn't mean it's the only use, and that all examples must be directed toward that sole end.) Any Blocking spell can be cast as a Regular spell (
Krommquote), taking a Concentrate Maneuver and treating it as though it had a one-second cast time. (That is, Blocking spells are not "free actions", even when cast as a defense.) In these cases, you might allow a bit more flexibility in the wording -- but even with a full second, it's still a very brief, straightforward command. So no need to dust off the well-rehearsed wish and demon-compelling contract arguments there, either.