The regional impact of transform seismicity on ridge-crest hydrothermal venting and ridge-transform dynamics is investigated using a June 1–7, 2000, Blanco Transform earthquake sequence. The mainshock (Mw 6.2) and 170 foreshocks and aftershocks were located using T waves recorded on U.S. Navy hydrophones, and indicate that the active transform fault may be farther south than in previous tectonic models. During the earthquake sequence, two temperature probes were deployed in black smoker chimneys at the Vent1 and Plume hydrothermal fields along the southern Juan de Fuca Ridge. These two hydrothermal systems are ∼39 km northwest of the mainshock's acoustic location. Both probes show significant (>5 °C) temperature declines following the mainshock, the Vent1 temperature changes occurring over days to weeks while the Plume changes were coseismic. The Vent1 and Plume fluid temperature decreases are consistent with earthquake-induced changes to permeability in the upper ocean crust. The evidence suggests that deep-ocean hydrothermal systems can be altered by large earthquakes even at a distance and across tectonic provinces, and ridge-crest seismic and magmatic activity are not the only causes of change to hydrothermal systems.