Perhaps the Dark Curse is weakened due to it already being cast and broken once before….and some of the side effects that used to come along weren’t able to make it because the Dark Curse wasn’t as powerful as it once was during it’s first cast?
-Thus due to the Dark Curse being weakened…it no longer requires the Heart of the Thing you love most….but instead it could require the Blood of the thing you hate(Or LOVE because we don’t know Zelena’s feelings to Regina….she could love her sister, but still want her to suffer and take everything that was hers.) Therefore Zelena required Regina’s blood because Regina is the person she loves or hates the mosts.
-But My most loved theory is that The Wizard is Zelena’s father, and the one she loved most. Therefore she required his heart, and it would explain why The Wizard wanted Zelena dead. And she uses Regina’s blood for some terrible act that we’ll probably see in the Season Finale
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor