From the help file...

syntax: cast 'leech unholy blessing' <source weapon> <destination>
This spell allows an anti-paladin to transfer the unholy power a weapon,
either their own or another anti-paladin's, has accumulated to another
weapon.



Unholy power = unholy blessed weapon...

ie you cannot leech from a weapon that isnt unholy blessed, and you need to leech it into your own unholy blessed weapon!