How does Oberon intend to blackmail Titania into giving him what he wants in A Midsummer Night's Dream?
For the answer to this, you can look in Act II, Scene 1.
Oberon wants Titania to give him the changeling child, but she will not. So he figures out a plan.
He decides that he will have Puck go and get a certain kind of flower. Using that flower, Oberon will make a potion that can be put on the eyes of a sleeping person. When the person awakes, he or she will fall in love with the first thing that he or she sees.
What Oberon plans is to put the potion on Titania's eyes. He will then tell Titania that he will not lift the spell until she gives him the child.
Oberon intends to blackmail Titania into giving him the child by making her fall in love with someone/something else. So long as her love and undivided attention is on someone/something else (Oberon doesn't care who or what she falls "in love" with). And while she is distracted by her newfound love, Oberon intends to steal the boy away from her.
In Shakespeare’s play A Midsummers Night's Dream, Oberon is the Fairy King. He wants the Indian boy that Titania has in her possession. He is malicious towards Titania. Oberon wants to take the child from her since she will not give the child to him.
Oberon wants to make the child his servant, but Titania has been raising the child as hers since the death of her own child. To get the child Oberon casts a spell on Titania. The spell causes her to fall madly in love with Bottom. As soon as he gets the boy, he releases her from the spell, but he has done a cruel act to her.