The idea for this project was shamelessly copied from meskarune's i3lock-fancy.
It uses scrot to take a screenshot of the desktop, then ImageMagick blurs the image and adds a lock icon and text.
By using information from xrandr and basic math, this script supports multiple monitor setups, displaying the icon and text centered on all screens.
The lock icon is different from the original project, with a transparent black circle around it. The text is also an image, making it easier to customize (and to put it at the correct position). Finally, it uses vanilla i3lock instead of i3lock-color. The author of i3lock-color is not maintaining it anymore. If you want to customize the colors of i3lock, the recommended version of i3lock-color is this one, maintained by Chris Guillott.
Make sure you have all the dependencies:
sudo apt-get install scrot imagemagick i3lock
Copy the lock script along with the images to some place on your system (e.g.: the i3 folder) and give it execution permission:
git clone https://github.com/guimeira/i3lock-fancy-multimonitor.git
cp -r i3lock-fancy-multimonitor ~/.i3
chmod +x ~/.i3/i3lock-fancy-multimonitor/lock
Create a key binding on your i3 config file (in this example I'm using $mod+p):
echo "bindsym \$mod+p exec /home/<your username>/.i3/i3lock-fancy-multimonitor/lock" >> ~/.i3/config
Now reload the i3 configuration file. By default, the key binding is $mod+Shift+c.