Psalms 109:2-4 (KJV)
2 For the mouth of the wicked and the mouth of the deceitful are opened against me: they have spoken against me with a lying tongue.
3 They compassed me about also with words of hatred; and fought against me without a cause.
4 For my love they are my adversaries: but I give myself unto prayer.
When people are against us, persecute us, discredit us, tell lies about us, etc.. and all this because we are following Jesus not because of what we have done wrong then we are to pray for them. Here are some related verses:
1 Peter 3:16-17 (KJV)
16 Having a good conscience; that, whereas they speak evil of you, as of evildoers, they may be ashamed that falsely accuse your good conversation in Christ.
17 For it is better, if the will of God be so, that ye suffer for well doing, than for evil doing.
When we do the wrong thing then there are consequences to suffer. We should not do wrong but good and if we suffer, like being falsely accused, for doing good that is much better. Then we are to bless.
Matthew 5:44-45 (KJV)
44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
Romans 12:14 (KJV) Bless them which persecute you: bless, and curse not.
The way we bless is by praying for them and being kind to them in action and not retaliating.