Personally, I love them for ammunition. Not too impressed for melee weapons though.
Silver blanch allows you to bypass DR without the -1 to damage. Awesome.
Cold Iron blanch, meh. Unless I specifically want enchanted ammuntion intead of the weapon itself, I'd rather just pay the extra 1gp/20 arrows.
Adamantine blanch - SIX times cheaper than actually buying the stuff. (for ammunition anyway)
The good news? They DO stack with the base material. The bad news, they don't stack with each other.
So, I get to fill my Efficient Quiver with silver+coldiron arrows for only 36gp and would probably carry around 20 adamantine arrows for an additional 201gp.
Good for arrows, bad for anything else.
I agree. 10 hits with ammunition, 1 hit with a melee weapon? I'd seriously consider house-ruling something there.
Probably I would let the blanch cover 1 melee weapon or 50 ammunition, and have a duration of 1 hour. That way they're not simply a cheaper way of getting special ammunition, nor are they useless for melee.