The undead trait confers
Immunity to any effect that requires a Fortitude save (unless that effect also works on objects or is harmless). (Monster Manual, p. 317)
The spell Blast of Force has the following effect:
a successful hit forces the subject to make a Fortitude save or be knocked prone (size and stability modifiers apply to the saving throw as if the spell were a bull rush). (Spell Compendium, p. 31)
As being knocked prone is certainly not harmless, the question is whether you can target an object with Blast of Force to achieve a bull rush effect. The spell reads:
You must succeed on a ranged touch attack with the ray to strike a target.
There is no mention on having to target a creature, so you could use it to target an object.
Does that mean you could knock undead prone with Blast of Force?