Now who are these five?
Come, come, all children who love fairy tales.
Assuming Feth has a 5+2 green block previously activated. If he draws a Green 1, can he use Purple 1 poem's ability to set a card +/-1, to set as Green '0' and place on 5 or 2, to form a 'new' block of 7 Green?
Actually would be 2 questions in one, ie can blocks be re-scored (for other hearts/new ravens) and can cards be set to 0?
Cards cannot be set to 0 (that I know of).
Blocks can be re-used, though, if you for example discard the green 2 with a red 1–3, and then set another green 2, that's another activation.
Also, IIRC, Purple poem cards don't do a ±1; it's another colour that does that. Blue or green, I think.