What does Origen mean in his response when Celsus accuses Christ' of using black magic when performing miracles? What is the actual argument?

No need to consult Wikipedia when one has Olson's book, from which only a partial quote is given above, from his whole chapter on Origen. Pages 99 to 112 are headed, 'Origen of Alexandria Leaves a Troubling Legacy' but not with regard to his response to Celsus on this point in question. Let me give more fulsome quotes to show what Olson was pointing out.

"He took on the Roman philosopher Celsus, who was attacking Christianity as ignorant and superstitious, in a devastating response to On the True Doctrine titled Contra Celsum, or Against Celsus. More than any other single Christian apology, this book of Origen's defeated a Goliath of opposition to Christianity and ushered the young religion into a new age of respectability in spite of continuing persecution.

Origen's aim in his writing career was 'to provide Christians who raise intellectual problems with answers in accordance with Scripture, so that they do not go and seek them in great gnostic sects.' [Henri Crouzel, Origen, trans. A.S. Worrall (San Fransisco: Harper & Row, 1989) p.14] He was apparently quite successful...

He left an ambiguous legacy of both intellectual greatness, and confusion for later Christian thinkers to wrestle with. On the one hand, his profound arguments and insights helped the educated classes of the Roman Empire take a view of Christianity different from Celsus's. They began to look upon Christianity as a viable philosophical and religious alternative to the numerous competing visions of reality and spiritualities within the empire. On the other hand, Origen's explanations of Christian doctrines such as the Trinity and person of Jesus Christ set the stage for great controversy one hundred years after his death." The Story of Christian Theology pp 101 & 102, Roger E. Olson, Inter-Varsity Press, 1999.

Having set the scene with a balanced view of Origen, Olson then deals with the question raised here.

"Against Celsus, then, Origen claimed that Christian truth is not completely in conflict with Greek philosophy at every point but that it rises higher in its concepts and especially in its ability to identify a particular historical manifestation of the divine goodness , Jesus Christ. A typical example of Origen's refutation of Celsus is his response to the latter's claim that Jesus Christ was merely a magician like many others and not God, as Christians claim. To Celsus, Origen replied,

'There would indeed be a resemblance between them, if Jesus, like the dealers in magical arts, had performed His works only for show; but now there is not a single juggler who, by means of his proceedings, invites his spectators to reform their manners, or trains those to the fear of God who are amazed at what they see, nor who tries to persuade them so to live as men who are justified by God.' [Origen, Against Celsus, p. 68]

"In the final analysis, Origen appealed to the historical fact of Jesus' resurrection, to the lives and deaths of the apostles, and above all to 'the manifestation of the Spirit and power' in the entire history of the people of God as proof of the truth of Christianity. It is apparent that for Origen there was no single proof or argument that by itself would sweep away Celsus's objections and skepticism. It was rather a series of ideas and facts that, woven together, constituted a strong case for Christianity's truth and superiority over the Greek and Roman philosophies and mythologies. At the end of Contra Celsum one has the sense that Origen has at least raised Christianity to an intellectual level alongside Celsus's 'true doctrine' (Greek philosophy) and made it impossible from then on for anyone to declare Christianity a folk religion fit only for the ignorant and superstitious." (Ibid, p. 102) [bold italics mine]

This helps answer the question: What did Origen mean in this explanation? Is there more to this argument? What he said about Christ working miracles to reform [not his audience, in general, but his followers in particular] was but one strand in his whole argument about just who Christ is. Yes, there was a lot more to his argument than just that one point. I hope this helps set Olson's explanation in context.