Hey @tmo! So you can create a custom e-mail sign-up prompt with just a little bit of JS. Add this to the Custom JavaScript section of the Customize settings and just replace the string in the message variable with whatever you want.
// Removing text from e-mail sign up (with custom message)
if (document.getElementById("emailsub") !== null) {
var esub = document.getElementById('emailsub');
var message = 'Subscribe to my blog please!'
var text = esub.getElementsByTagName('p')[0].innerHTML = message;
}
Hope that helps!