Blade Arts are a combat mechanic in Xenoblade Chronicles 2. They are effects that Blades can grant their Driver in battle. They happen automatically without player input. The equivalent of this mechanic in Torna ~ The Golden Country is Rearguard Arts.
Each Blade Art provides the Blade's Driver with a certain beneficial effect, ranging from increasing Driver Art power to drawing aggro. Blade Arts have a particular activation condition (e.g. missing an attack); if this activation condition occurs, the Blade Art then has a chance of activating. After a Blade uses a Blade Art, it cannot be used immediately again until a specific length of time (the cooldown) has occurred.
When a Blade uses a Blade Art, they call out an associated voice line. Unlike Driver Arts and Specials, this voice line is not the same as the name of the Art (for example, Debuff Cancel's associated voice line is "Revitalise!"). The voice line for Draw Aggro is unique to each Blade that has the Art; the others are the same across all Blades.
The Poppi forms can choose which Blade Arts they effectively have using Arts Cards in Poppiswap. Each Arts Card corresponds directly to a specific Blade Art.
As Blade Arts are passive and their effects tend not to majorly affect combat, they typically do not affect mid-combat tactics, and for that matter may be mostly ignored in setups at all but the highest levels of optimisation; the Specials and Battle Skills of a Blade generally play a much greater role in battle than the Blade Arts do. For the most part, they may be treated simply as a bonus when they do happen; adapting tactics to accommodate them is unnecessary.
The one situation in which it is worth actively considering Blade Arts is in setting up Poppi, as some such Arts are more useful than others. AC System Reset (corresponding to Debuff Cancel), AC Reaction Nullify (Nullify Reaction), and AC Arts Boost (Arts Plus) in that order are the most useful in general. In situations where defense may be entirely forgone, AC Critical Up (Critical Up) and AC Recharge Boost (Recharge Boost) are the next most effective at increasing damage.