Imagine the wall of force effect as an actual wall
Most walls are solid barriers, and spells that create effects often can't create their effects through walls. Spell Failure says, "If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted."
The spell blade barrier creates an effect that, in this case, is "a ringed wall of whirling blades with a radius of up to 5 ft. per two levels [that's] 20 ft. high." Casting a blade barrier spell so that its effect overlaps with a low wall either causes the spell to fail or, more generously, causes the blade barrier effect to wrap over the low wall… not go through the wall.
That is, the blade barrier effect can't be shaped—in this case only its radius can be picked by the caster—so part of the blade barrier effect must be through the wall. Yet the blade barrier effect must stop at the wall and be on both sides of it. Thus the blade barrier effect is no longer contiguous, a necessity for the effect's successful creation. The GM may rule that the blade barrier effect conforms to the wall, wrapping around or over it (yet not through it), but a GM may instead rule that—because the characteristics of the spell can't be made to conform to the wall—the casting the fails, and the spell's wasted. In either case, though, the wall (or a wall of force effect) would remain unharmed by the blade barrier effect.
Essentially, what's being tried here is like trying to summon a Huge (tall) creature into a 15-ft.-by-15-ft. space that's split by a 10-ft.-high-yet-paper-thin wall: from several vantages the caster may have line of effect to all of the squares individually (see Aiming a Spell on Line of Effect), but the spell's overall effect can't be made to conform to the space into which the caster wants to put it. Trying to do that typically just ends up with the spell failing.
This is different from an area spell that's a burst or spread like the fireball spell (see Aiming a Spell on Area). The caster of the fireball spell picks a grid intersection as the spell effect's point of origin and counts squares from that point of origin to determine what's caught in the spell's area. This allows a fireball spell that's cast up in the air to affect stuff on both sides of a low-lying wall… and the wall.
A caster doesn't need a fireball spell's area to be clear of obstructions for it to be used—that's handled by the burst mechanics—, but a spell that creates an effect over an area requires more delicacy in its grid placement than a burst.