I doubt you will find many knife laws that say how to determine the blade length. It's one of those ambiguities they like to use to keep us on the raw edge of legality, never sure what we can get away with.
The only fair measure would be
penetration depth, how far into a ... an object ... the blade would go:
from tip to bolster. By that measure, as well as I can see from your picture, the blade is precisely 3.5".
I also doubt that quibbling over a few technical details would, ultimately, matter -- like a recurve or hawksbill actually having a longer cutting edge than a wharnecliffe, or a thumbstud preventing the blade from sinking all the way to the bolster.
We've also heard simplistic and unfair measures used, like placing the blade across a palm and declaring it too long, by a person with narrow hands ... or the LEO who tried to say the three-inch rule referred to overall length. Good grief.