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 below, it now splatters the sky. Once, through my lens, I saw a painter on Earth whose artistic style of paintbrush-flicking now manifested itself in a cataclysm.
See the humans at the pointy building run, run, and the visitors on the green sand disappear into caves! See how the blob — probably a majestic beauty in the deeps — divides, doubles, dirties the image of Earth: one's telescope would see a little less of the planet with each second. Asexual reproduction, and terribly rapid growth makes the rich air a fertile bed for the black blobs.
Those four islands are hidden by the spreading black mould, which festers exponentially. Orange bombs, sent by humans closer to the water's edges, are too late and too feeble. Before long, a spongey black gunge has populated the entire waters, absorbing, I'm sure, life in its shadow (without the Sun, is all life on Earth demolished? All but one species, in this case).
Looking again through the telescope — that is, if anything happens to see this event — regard the Earth, now black and dead. Unlike light, sound does not travel in a vacuum. Our ears ought to be incredibly grateful for that, I fear.