When a monster selected for “Amazoness Blowpiper”’s effect is flipped face-down, the effect is reset.
After selecting a monster with “Amazoness Blowpiper”’s effect, if “Amazoness Blowpiper” is flipped face-down or removed from the field, its effect still applies until the end of the turn.
Netrep Rulings:
“Amazoness Blowpiper” may only target a face-up card.
Netrep Q&As:
Q: Can “Amazoness Blowpiper’s” effect be used more than once per turn?
A: As per the card text, it says you use the effect once during each of your Standby phases. That is the only time the effect is used. Curtis Schultz, 12/08/2003