The lightyear is an unfathomably long measure of distance to human minds, and it really is an odd trick of slow travelling light that those in a far-off solar system could only learn about the befouling of Earth several centuries (to use a human unit) after it actually happened.
But anything out here, with a magnificent telescope perchance trained on Earth, could have seen that blue planet turn black very rapidly. So rapidly, in fact, that it might be considered a cosmological anomaly at first. Soon afterwards, examination of the footage would reveal a stranger fate for that semi-spherical harbour of semi-intelligent life.
Had something looked — and remember, things further away may still look in the future — it would have seen the following:
In the largest patch of blue water, there are four islands near the centre of the blue. Not far from a southerly beach on the southernmost island, popular with incoming visitors because of its green-coloured sand, a square pointy building stands, with a planked pier extending into the water like a finger.
A dark green spike emerges from the water, and then the rest of a capsule climbs out from sea level, besides the pier. Two people, entirely clad in black, eventually exit the vehicle, carrying a bright yellow case. It is sealed, although its contents squeeze on the case walls, repeatedly changing the centre of mass (thus causing the two carriers' awkward stumbling).
Next, with a particularly forceful yank, the case seems to pull itself upwards. See now the carrier on the left wobbling on a loose plank. Watch that fateful case — oh! how inconsequential was Pandora's box! — fall onto the steel stretcher, and watch the seal crack as a black goo leaks out, dropping in a solid, liquid blob onto the pier. Not only does it resist the clumsy hands that try to pick it up in a panic, but it also slides off into midair.
Humanity ended itself that day. Curiosity (for I suppose it was curiosity) about this creature encouraged them to pluck it from the deep, mysterious waters and expose it to the drier world above, where, as a blob released from the claustrophobic pressures of the ocean depths, it expanded exponentially, consuming everything in its path until the entire planet was transformed into a dark, writhing mass visible from space.