There are two ways to do this.

1. The login screen background blur is part of the acrylic design that Microsoft is adding to various UI elements on Windows 10.
   Open the Settings app and go the Personalization group of settings. Select the Color tab and disable the ‘Transparency effects’ by turning the switch off. This will disable the acrylic effect all through the OS and that includes the login screen.

2. If you just want the login screen blur to go away and keep the rest it is a Registry edit that you have to do.

Go to the registry key

Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System

Right-click the System key and select New>DWORD (32-bit) value. Name the value DisableAcrylicBackgroundOnLogon. Double-click it and set its value to 1. That will do the trick. The blur from the login screen background will be removed but you will still see the acrylic effect in the rest of the OS.