What is the opinion of Jewish authorities on Magic, particularly black magic? Traditionally Jews have denied that anything happens outside of the will of God, yet Deuteronomy 18:10 states that these practices do happen. What is going on?
This is a matter of debate among the medieval Jewish authorities.
Maimonides believed that magic does not exist, and explained the verses that prohibit witchcraft and the like do so precisely because it is meaningless nonsense.
Maimonides, Laws of Foreign Worship 11:16 (Simon Glazer translation, available at sefaria.org):
All of these things are false and spurious, and it was with such that the ancient idolaters misled the peoples of many lands so that they be following them. And it is unbecoming to Israel who are exceedingly wise to be attracted by these absurdities, nor to even imagine that they are of any consequence, even as it is said: "For there is no enchantment with Jacob, neither is there any divination with Israel" (Num. 23.23); and it is again said: "For these nations that thou art to dispossess, hearken unto soothsayers and unto diviners, but as for thee the Lord thy God hath not suffered thee so to do" (Deut. 18.14). Whosoever believes in these matters, and their like, and suppose that there is wisdom and truth in in them, save that the Torah disallowed them, such are none other save from among the foolish and ignorant and are to be included among women and children whose mind is not sound. But wise and sound-minded people know that all these matters which the Torah disallowed are not matters of wisdom but formless nonsense followed by senseless people for the sake of which they abandoned every path of truth. Even because thereof the Torah, in admonishing against all these vanities, commanded, saying: "Perfect shalt thou be with the Lord thy God" (Ibid. 18.13).
And now, know and understand regarding magic, that the Creator (may He be blessed) created everything from nothing and made the upper realms the guides of what is beneath them; and He placed the power of the earth and all that is in it in the stars and constellations according to their motion and direction, as has been demonstrated in the science of astrology . . . However, it was one of His great wonders that He placed within the upper realms alternate ways and forces by which one might change the governance of the realms beneath them . . . But it is the regular governance of the constellations that the Creator (blessed is He) desires, which He placed in them to begin with, and this would be the opposite. This is the secret of magic and its power, such that the rabbis said regarding magical practices that they “contradict the Council Above”; in other words, they subvert the simple forces of nature, which is a contradiction to the upper realms to some extent. Therefore, it is proper that the Torah prohibit them so that the world will be left to its normal function and its natural state, which is the desire of the Creator . . .
In other words, magic is a way of subverting the normal rules of nature, using an alternative system still (ultimately) created by G-d.
לחכמים ולמכשפים: אחשוב שהחכמים היו האנשים שהיו חכמים בחכמת מצרים וענין החכמה ההיא היתה להמציא חכמת הכישוף ויעשו בה פעולות זרות אין מדרך הטבע שיגיעו ממנו וזה אם כשיאחזו העינים ויביאו לחשוב שיעשו מה שאינם עושים אם בשימציאו התחבולות הטבעיות יתחדשו בהם ענינים זרים ידמו מפעולות הכשפים או בשיעשו אלו הפעולות הזרות על דרך הכשוף אם היה אפשר שיתחדשו בדרך הכשוף כמו אלו הפעולות על דרך האמת והנה לא נודע לנו עד היום מהות זה הכשוף וענינו ולזה לא ראינו לדבר בו והמכשפים היו האנשים שהיו מתעסקים בפעולות הכשוף
To the wise men and to the sorcerers: I think that the "wise men" were the men that were wise in Egyptian wisdom. And the topic of that wisdom was to produce magical theory and perform with it strange actions that would not naturally result from it. And this is either by sleight of hand and bringing [people] to think that they did something that they didn't [actually] do, or by producing strange things that are similar to magical activity [but] via the utilization of natural methods, or by performing these strange actions via magic if it is possible in truth for such actions to be generated via magic. And behold, until today we do not know the nature of this magic and what it is, and therefore we have not seen [fit] to speak about it. And the "sorcerers" were the men that engaged in magical activities.