hi,

If i block someone, can they see me online?

Thanks

See here.

Information on blocking people

If you remove from contacts a previously authorised person, can they:
Q1) See you online?
A1) Yes they will still see you online

Q2) Contact you?
A2) They can contact you if you have either allow all contact or allow those you have authorised, but not if you have 'contacts only' and each of these can be set separately for calls or chats.

Q3) Blocked Users
A3) blocked users cannot contact you or see you online in any circumstances unless you have enabled your status to be shown on the web, that URL is not, and can not, be filtered by Skype username as per the privacy options.

Q4) Will other people notice, that I have removed them from my contact list?
A4) No, they won't

When you block somebody, they goes back to ?mark!
Until it’s removed from this list, a Skype user who is blocked will be unable to see the status of or communicate with the user who blocked them.
Users who have been blocked may be unblocked at any time by the user who put the
block in place, simply by removing the blocked user’s name from the blocking list and re-requesting authorization again.

With regard to authorisations:
Authorizations are stored on both sides. So if you had already asked this contact for authorization previously, his/her client has remembered it, and if you now ask again, the response is returned automatically without popping up anything for the remote side, since you already have been authorized previously.