[Raynor]

How do you come to that conclusion?

Melkor used his power and spread it through the matter of Arda in order to subdue and corrupt; his whole being and deeds were bent to that purpose. I doubt that this power can be directly used for good. This power continuously erodes even the mighty hroa of the elves, which were supposed to endure until the end of the world. My argument is that even if the melkorian element isn't strong enough to corrupt physically/directly, the moral choice of using it, per se, denotes corruption. I believe that using it is perpetuating it; we could argue along the lines of Gandalf purifying this, but that would mean returning Melkor's power to its original status, which is another situation altogether. The finnishing passage of the Silmarillion might be telling:

Yet the lies that Melkor, the mighty and accursed, Morgoth Bauglir, the Power of Terror and of Hate, sowed in the hearts of Elves and Men are a seed that does not die and cannot be destroyed; and ever and anon it sprouts anew, and will bear dark fruit even unto the latest days.

Even words, information, which come from Melkor perpetuate his work of corruption; I would say, even more so, his lasting influence in the matter of Arda.

So all Númenórean ‘magic’ was done by members of the Line of Elros? I suppose that idea could work, barely.

We should also keep in mind that Eonwe gave to all the race of the numenoreans "wisdom and power and life"; I think it is safe to say that if the Numenoreans were able to do magic, this is a good source of it.

Well, we have Beorn and his skin-changing. And the Drúedain are also interesting; they have no Númenórean blood but according to ‘The Drúedain’ in Unfinished Tales they ‘had, or were credited with, strange or magical powers’.

Well, I would first notice that not of these examples refer to normal Men. Beorn might be an exception a la Tom Bombadil (I don't think his abilities were inherited by his descendants, I will have to check on that). The story of the statues from UT is really intriguing; do they necessarily point to magic? I wouldn't say so. I would also note that the druedain were eligible for becoming numenoreans too; perhaps, a gift of power was expressly given to them too - or maybe simply their moral choice elevated them (we know that at least the contrary is true - wrong moral choices diminishing one's status).