score:12
Their website is very clear.
Once you clear customs and enter the arrivals hall, the car rental shuttle parking space is right outside – next to “Flybus Parking”. This shuttle will take you to the Rental Car Area where Blue Car Rental will be your second stop on its route. Quite convenient and easy to find.
The shuttle runs from 06:00 – 18:00 every day with approximately 15 minutes intervals. If you are picking up after 18:00 we ask you to walk over to our delivery office. Simply exit the airport on the departure side and there you can see our delivery office across the parking lot.
Our delivery office at the Keflavik International Airport is located within 4 minutes walking distance from the airport at Blikavellir 3. Renters are also more than welcome to take the walk over to us during the day.
I've been there last Christmas and I confirm what they say. If it's a nice day, it's a very short walk (400m?) otherwise there's a shuttle bus connecting Airport and parkings (where the car rental is). There're also other private vans taking people to other car rentals further away, don't jump on one of those!