What does blocking a number actually mean?

I've got an iPhone five and have a group iMessage with about 5 of my friends. Obviously when my internets not working or my 3G isn't on my iMessages go through as texts. One girl out of our group chat is going on holiday next week (abroad) so I really don't want to be accidentally sending her messages and causing myself to spend a fortune. Will blocking her number stop texts going through to her or just texts from her coming through to me?

It's only the person that goes abroad that gets charged for texts.

Yes it will.

Your a doofus. No gets charged if your have the same network or service provider. Common sense, not unless you use a payasyougo phone then of course you get charged.