If i play Feast or Famine and another player plays forgotten plans, can I take my +6 gold during the plot phase, but then still initiate challenges during the challenge phase as the text of my plot card is now blank?

If the plots text box is blank then you wont be able to get the +6 gold. but yes you will be able to initiate challenges.

Mighty Jim said:

No. Feast or Famine creates lasting effect. Blanking doesn't matter.

Here you go.