Part of the look of Time Spiral's black-aligned environments is the dripping carcasses of Phyrexian horrors. When Karn, Silver Golem became the manifestation of the assembled Legacy Weapon and blasted Yawgmoth to smithereens, Phyrexian invaders of all shapes and sizes perished throughout Dominaria. What you see in the background of many Time Spiral black cards represents the viscera and vehicular wreckage of those Phyrexians.
From the Time Spiral style guide: Phyrexian remains
Check out the environment around Gorgon Recluse or the volcanic battleground depicted in Molten Slagheap.
Gorgon Recluse art by Darrell Riche
Molten Slagheap art by Daren Bader
In addition, every basic Swamp in Time Spiral depicts Phyrexian bio-wreckage extremely well. Check out the toxic sludge-fens in these Swamp pieces. You can see hulls of ships, organs still sluicing ichor, even claws or ribcages of individual invaders that almost feel like they're about to get up and resume their destruction of life on Dominaria.
Swamp art by Craig Mullins
Swamp art by John Avon