Warren Torchmaster

You don't choose a target for Warren Torchmaster's ability at the time it triggers. Rather, a second "reflexive" ability triggers when you blight 1 this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

If you can't place -1/-1 counters on any creatures you control (probably because you control no creatures), you can't choose to blight.

All of the -1/-1 counters must be put on a single creature. You can't choose to split them up between multiple creatures.

If a creature that has +1/+1 counters on it receives enough -1/-1 counters to cause it to be destroyed by lethal damage or put into its owner's graveyard for having 0 or less toughness, effects that refer to the counters on that creature when it died will see all of those +1/+1 and -1/-1 counters.

The creature you choose to put -1/-1 counters on doesn't have to have enough toughness to survive the process. For example, if you blight 2, you can choose to put the counters on a 1/1 creature you control.

If a creature has +1/+1 counters and -1/-1 counters on it, state-based actions remove the same number of each so that it has only one kind of those counters on it. For example, if a creature has three +1/+1 counters on it and two -1/-1 counters are put on it, state-based actions will remove two of each of those kinds of counters, leaving the creature with just one +1/+1 counter.