How do I read messages from a blocked number?

You don't.

You have to unblock and then you can read the messages that comes after the unblock.

You don't get messages from a blocked number at all.

kakeysha wrote:

How do I read messages from a blocked number?

What the point in blocking them if you can still read what they sent you?

That defeats the purpose of blocking someone.

kakeysha wrote:

They don't go to spam or anything?

There is no spam folder for messages. They are either delivered and received in your messaging app, or not. When you block someone, their messages are simply never delivered into your account.