Remove blur effect from lock screen on High Sierra
I found this article online, and I hope this helps. I just saw someone who had their lock screen be un-blurred, and I started researching it. So, I saw this and created an account on StackExchange to post this. I hope this helps. I am also new to StackExchange, so if the answer could be changed for the better please let me know so I can change it.

Here is the link

If you’d rather your login screen not be blurred, you can do that too! I recommend first adding a custom image to your desktop so that there is an image in so that there is an image in the /Library/Caches folder.Then, copy that image to your desktop by dragging it there.



We’ll use this image in a little bit. First, open whatever image you’d like to show up behind your login screen with Preview. Select the entire canvas (Command+A), then copy it (Command+C).



Next, head to your desktop and open the file you copied from /Library/Caches. Paste (Command+V) what you just copied.



You will need to move and resize what you’re pasting quite a bit in order to get everything just right. When you’re done, save the image (Command+S). Head back to your desktop, and drag your edited image to the /Library/Caches folder. You will be asked if you’d like to replace the file that’s there; click “Replace.”



You’ve now placed your custom background image. Go ahead and log out of your user account to see how it looks:



It worked! In my case, however, the white maple leaf is making it hard to read the white text used by macOS. To fix this, I found a different version of this image where the leaf is blue.



Much better! This version puts the darker part of the image behind the text, which makes it easier to read everything. You’ll probably have to experiment with this a little yourself, but to avoid problems favor dark images over light ones, and simple images over complex ones.

If you’re a Photoshop guru, you might prefer making an image yourself over messing around with Preview in this way. You can do that, but the image you create needs to meet specific criteria. It must be:


A PNG file
The same exact resolution as your primary display
RGB, sRGB, or Adobe RGB color mode.
8bit colors
Unlocked background layer
Named “com.apple.desktop.admin.png”
In our tests, even doing all these things can still be hit-or-miss, so we suggest using the Preview method. It’s just simpler.