Blesta will send along the correct IPN URL when a payment is made from Blesta. Setting the IPN URL in PayPal seems to the the only way to RE-ENABLE IPN, if PayPal disables it from failed attempts.
So, the question is: Is the customer making payment through Blesta? ie, clicking the PayPal button to pay during checkout? And if so, is IPN enabled in PayPal?
Lastly, you might check your HTTP logs to see if there are any POST requests from PayPal's IPN, and look at the gateway log in Blesta - Tools > Logs > Gateway.