Paul Farquhar
Legend
The "Blurred Movement" ability is the thing that matters. But RAW says it has no effect on defensive CR, and who are we to argue with rules?How do you calculate it? I come to offensive CR 5 and defensive CR 1/2, not counting the disadvantage on attacking them.
Base defensive CR is 1/8, which is increased by one pip for having more than 2 AC over the line (note that it's one for every two AC, round down, I suspect this is why you are getting 1/2 rather than 1/4). Common sense says Blurred Movement is worth at least +1 pip, and therefore final CR is at least 2, but rules say no.
But this creature is a good example of why CR can never work. If you have the right abilities, e.g. Toll the Dead, Reckless Attack, spells that don't target DEX, then their low hp makes them pushovers. They are even vulnerable to Sleep! But if all you have are whiffy melee attacks these things will punch well above their weight.