Well, it was intended to just kill people initially though that wasn't how it ended up playing out and it did end up harming the land based on what we saw in the Tales from the Shadows chapter Worlds Forsaken) https://na.finalfantasyxiv.com/lodes.../#sidestory_03)

"Black Rose wrought immeasurable destruction to organic life-forms, resulting in widespread environmental upheaval. The ambient aether ceased to flow along its normal channels and stagnated, rendering the land barren and uninhabitable. Inevitably, the wild imbalance of natural energies affected neighboring regions and, in some cases, led to sudden and unpredictable alterations in flora and fauna. Overnight, the crops on which man relied for sustenance became toxic, further increasing the death toll. In the aftermath of these unprecedented disasters, the communities formed by sentient beings fell into disarray, and all forms of societal organization, from small settlements to entire nations, were abandoned as individuals struggled desperately to avoid annihilation.

It did not take long for the cascading environmental repercussions to be felt in Garlean territory. Due to the aftereffects of Black Rose, the ceruleum that once supported daily life in the Empire and powered its magitek armies failed to combust. The machines duly fell silent, bereft of the energy required to set them in motion."

During the latter half of Stormblood we kept seeing NPCs referring to a 'thinning of the aether' in many places, which in Shadowbringers we found out was a result of the increase of Umbral polarity aether leaking in from the First. As we know from the events from the Events of the Eden that Umbral aether tends to suspend things in place. All it really took was an extra push from this extra aether to cause damage to everything around it which was enough of a push to cause the calamity causing the flood of aether to cause Black Rose to spread like a plague.

It is pretty much the reason that even after 200 years civilization has not recovered as the land has still not worked out to restore the proper polarity between Astral and Umbral during that time.