Shoutbox

If you have blocked a contact then they cannot see any of your updated statuses or activities, regardless of whether they are using Messenger Plus! Live.

She could've found out that you'd blocked her by asking a friend who has you on their contact list if you were online.  If you were appearing online to that person, but offline to your friend then they'd know you'd blocked them.

This is nothing to do with Messenger Plus! Live nor any other piece of software.  Block checking software doesn't work, the method I described above is the only way to know