What you can do is, you can create your own Login Page and customize it in whatever way you want.
You can retrieve the User Object by the Email given by the Client and then, you can try to check whether the user is Blocked or not, and depending on the result you can show the appropriate Message.
Let me know, if you face any issues,
Hope it helps!!
Hi, it seems you are looking for an option to change a system text. This is possible via te App Settings.
Please have a look at the following page; https://docs.mendix.com/refguide/system-texts/
The answer given by Jeroen was something new to me and was amazing. However I have faced the same issue and I have solved it by creating a nanoflow that check the user for blocked or not and will through the validation message of my own.
Hope my answer helps.