After upgrading my MacBook Pro to OS X software update 10.9.3, I instantly had an issue with my display. I have a monitor connected to my MacBook Pro that I use at my office. I connect the monitor using a mini DisplayPort cable which I plugin to the Thunderbolt port on my Macbook Pro. After the update, my screen appeared blurry. I could not figure out what was causing it because the menu for my monitor appeared sharp but my display was blurry.
The 10.9.3 update included additional support for 4K displays. Though my display is an Asus PA279 Pro Art display which has a max resolution of 2560×1440, this update seemed to cause issues with the way I had my display configured.
Here is what I did to fix the blurry image. I went into my monitor settings and changed the color mode from Standard to sRGB Mode. Most monitors should have this setting. You can toggle through the mode options to see what works best for your monitor. Alternatively, you could also recalibrate your monitor using a calibration tool if you have such a thing. I actually do have one but have not gone as far as that yet.
This fix worked for me.
I had the same problem when updating a Mac Mini which was connected to an Asus 27″ monitor. I had to change the color mode to sRGB and it fixed the issue.