Anytime you add the text block it automatically kicks off any text package that you had. You should still be covered for any usage you had before the block was added but there is a chance it may have prorated the usage.
An alternative to just blocking the messages is to bump up the text package to the next level. The next one up from 500 is 1500 and is only $5 more.
If you do end up with an overage charge on the bill due to prorations, call customer service and they should be able to work with you on fixing the bill.
Hope this helps!
When I grow up I'm going to be as smart as Danilove13 Very nice response. We need to clone you have you at every call center!