Why can I send him a text message when the other person is dark?

You can send a message to the other party when the other party blackmails you, but the other party cannot receive your message. When you call him, the number you dialed will also be busy. Every time you call him, it's this hint, which means that you have been blacked out by the other party, so even if you can send him a message, he won't receive it.