Suppose, I'm attacked by one creature. I block it with a Torch Fiend, and after I get priority I activate its ability to destroy opponent's artifact: pay red mana and sacrifice it. Is the attacking creature considered to be blocked or not?
thekingofthewalrus / adam prince of eternia is correct
However, do take note that if you sac the field after blocking (which still happens during the declare blockers step), neither it nor the creature being blocked will deal nor receive damage in the subsequent damage step. This could be relevant if the attacking creature has an ability which looks at the damage it deals (ie: lifelink) or a trigger which fires off when it's dealt damage (ie: boros reckoner). Be aware as well that if the attacking creature has trample (and wasn't also wasn't blocked by any additional creatures), all of its combat damage will still be dealt to the player (or planeswalker) it attacked despite being blocked, as no damage will be dealt to and absorbed by the blocking fiend which would otherwise (under normal circumstances) reduce that damage trampling over to the player/planeswalker by one (or whatever its toughness is at the time it blocks and receives damage). Lastly, note that as the fiend's controller you couldn't simply sac the fiend (after say blocking a lifelinker to prevent its controller from gaining life) unless there's a valid target (artifact) which allows you to use that activated ability. I hope this clears up any subsequent related questions you may have.
Anyhow, the best I can come up with myself is a game in the top 8 of a PTQ back during Urza block in which we were starting game 3 with time already expired, so the tiebreaker rule was that whoever had more life after 3 turns would win. And I lost to... healing salve.
Shout out to commandercast for keeping me entertained.
However, do take note that if you sac the field after blocking (which still happens during the declare blockers step), neither it nor the creature being blocked will deal nor receive damage in the subsequent damage step. This could be relevant if the attacking creature has an ability which looks at the damage it deals (ie: lifelink) or a trigger which fires off when it's dealt damage (ie: boros reckoner). Be aware as well that if the attacking creature has trample (and wasn't also wasn't blocked by any additional creatures), all of its combat damage will still be dealt to the player (or planeswalker) it attacked despite being blocked, as no damage will be dealt to and absorbed by the blocking fiend which would otherwise (under normal circumstances) reduce that damage trampling over to the player/planeswalker by one (or whatever its toughness is at the time it blocks and receives damage). Lastly, note that as the fiend's controller you couldn't simply sac the fiend (after say blocking a lifelinker to prevent its controller from gaining life) unless there's a valid target (artifact) which allows you to use that activated ability. I hope this clears up any subsequent related questions you may have.