A weapon with a special ability must also have at least a +1 enhancement bonus.
This is a rule for all magical weapons. Barring a more specific line of text that removes this requirement, it applies to all magical weapons, regardless of source.
Restrictions on arcane pool, it's hazy if they would apply to the Fiendblade ability (as while it uses points from the arcane pool, it is it's own separate ability and not subject to restrictions on that ability, at least, the ones written into the 'use a swift action to do blah' bit), but the rule on magic weapons is a global rule that applies to all magical weapons.
Technically by the rules on pre-requisites though, if you have a thing but not a pre-req, you lose access to the thing until you get the pre-req. So by a reading of RAW (to 'have', not to 'grant') and (gaining a pre-req means you regain the use of the ability you lost due to losing the pre-req), creating an unholy fiendblade (which has no unholy property due to not having the +1) and then casting Magic Weapon on it would give you the use of a +1 Unholy Fiendblade, at the cost of a swift action on your part and a standard action on your (or someone else's) part.
But the Fiend Blade does have a +1. In fact at 7th level it has a +2. (user: Doomglare)
To clear up any confusion, the Fiend Blade ability allows tiefling maguses to create a fiendblade with an enhancement bonus - and after level 5 trade out that enhancement bonus for a short list of special abilities. There is no limit on this in the text of the Fiendblade ability, allowing a Tiefling Magus to trade out all the +'s and end up with a weapon that cannot 'have' the special abilities the fiendblade ability has given it, due to not having the pre-requisite +1 enhancement bonus.