Diagnosis
The alternating red and blue flash is the electronic indicator's "end of life" warning for the filter. It's not an error code, it's just telling you the filter has reached its 100-gallon capacity (or the 2-month timer is up) and needs replacing.
How to Fix It
Install a new PUR Ultimate or Standard Faucet Filter (like the PFM400S or compatible). After you click the new filter into the housing, you must press and hold the reset button on the monitor for about 3 seconds. This clears the memory and starts the new countdown. The light should go solid blue or green.
If That Didn't Work
If the light keeps flashing after a new filter and reset, the battery in the monitor is likely too low to power the new cycle. You'll need to replace the CR2 battery. Pry off the battery cover on the bottom of the monitor with a small screwdriver.
Heads Up
The monitor works on both a timer and a gallon estimate. If your water is very hard or has a lot of sediment, you might hit the gallon limit well before the 2-month timer goes off, triggering the light early.