What it does
I'm not aware of "Blight" being an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules as a keyword mechanic, and I don't want to invent rules text that could mislead you at the table.
You might be thinking of a few similarly-named things: Blightsteel Colossus (a creature with infect), Wither (a keyword where damage is dealt as -1/-1 counters), or Infect (damage to players as poison counters, to creatures as -1/-1 counters). There's also flavor-named cards like Blighted lands (e.g., Blighted Woodland) that aren't a keyword but a card-name theme.
If you can tell me the specific card or set where you saw "Blight," I'll give you an accurate, Commander-focused breakdown of how it works and where it matters.
