The short answer: The onBlur function needs just a Msg (ie a Msg constructor that takes no arguments) rather than String -> Msg like your PostIssue constructor which needs a string in order to become a real Msg.
The long answer: onBlur does not have access to the value in the input field, so it’s not the way to access what the user has typed in. To get the current value in the input field you’ll want to use onInput (defined in Html.Events and store the value in your Model (in something like a currentIssue record field). You need to store the value after each input event (ie keystroke) since the view function needs to know what to render - if the input after each keystroke is not stored in your model, any new input in the field won’t be rendered. The example in the Elm Guide shows you how to set up a basic text input.
If you want to treat the blur event as a sort of submit action, you should create a separate Msg constructor that takes no input for use with onBlur. Then in your update function, when that message is received it can look up the stored value in your model for the current input and add it to the list of issues (or otherwise handle it as desired).