Send the wrong message to your predecessor and speak your mind.

If you send a blessing to your ex, but later find yourself regretting it, you can consider replying to a message telling him/her that you sent it wrong. This is an acceptable way, because it can not only express apologies, but also not cause too much trouble to the predecessor.

However, you should also consider the feelings of your predecessor. If he/she doesn't receive your blessing, then your message may be meaningless. Therefore, you should carefully consider your decision before sending a message.

Generally speaking, you should be respectful, honest and considerate when communicating with your predecessor. Although your relationship is over, you should still respect his/her feelings.