Blindsight can detect obstacles, and so it can detect a wall of force.
As for the question about painting and spider climbing, there's no "official" answer that I'm aware of, but I'll offer my thoughts. A wall of force cannot be affected by any physical effect. If you dab paint onto one, the paint slides right down to the ground and pools there. If you cast spider climb, you still can't climb it, because you can't interact with it. It's not "real" in every sense of the world. It's solidity without mass.
Blindsight (that works from hearing/sonar/radar/similar...but not smell) can detect it because physical forces and energies can still reflect off of the wall. You can get echoes from it in the same way that if you throw a rock at a wall of force, it's gonna bounce off. And, when it bounces off, it does so without losing any of its energy to the wall...in an apparent violation of physical law. Thus, it doesn't really "interact" with the wall in a thermodynamic sense. There's no imparting of momentum or heat to it in the rebound.
In fact, from that, one would speculate that a wall of force, being a perfect reflector for things other than visible spectrum light, would be far MORE discernable to blindsight than most barriers!
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I spend way too much time thinking about this kind of thing.