I have not read or watched Harry Potter but my brother has (and loved it). Real witchcraft is consorting with demons, and it seems to me in most of the books like Harry Potter if someone goes along with evil spirits and such things it is the villains. What is called real witchcraft today is usually paganism, though it is of course a step nearer to real witchcraft.
People have often used terms like witch and wizard and magic for fantasy events and abilities, since that is the most popular view of witchcraft, even among some Christians: that it is fantasy. Similarly elves and fairies shrank from evil spirits to friendly and even perfectly good creatures. The terms "black magic" and "white magic" appeared, and also some of the terms became more associated with one or the other: witches, warlocks, and sorcerers on one side, wizards, alchemists, and magicians on the other. Rowling apparently recognized at least that they were all the same, and her distinction between "black magic" and "white magic" was more soundly based on the actual morality of the act done with the magic than on merely shifting connotations of words. Doctor Strange, at least in the early comics, outright called his craft "black magic", yet again he actually battles evil spirits using power from "all natural sources" so to speak.
I agree with something I read in an article a while back: that Harry Potter can be dangerous if one doesn't know what real witchcraft is, or doesn't believe it exists, or doesn't believe it is wrong, and has an interest in the occult. Whereas if someone knows what real witchcraft is, and reads Harry Potter out of an interest in Fantasy, it is the same as reading any other good quality book by a non-christian author.
Since I believe there is real witchcraft I would not want to use terms associated with it in writing fiction, but even many Christians in western society "don't believe in witches", so I couldn't fault them on using these terms. Rather I would fault them on rejecting the existence of actual witchcraft, which is a separate subject from the merits of their books and art, and one problem that unbelievers share with many western Christians.
As a Fantasy writer I have been frustrated with the lack of terminology that actually refers to what Fantasy writers actually talk about: imaginary abilities and possibilities.
"This is what your folk would call magic, I believe; though I do not understand clearly what they mean; and they seem also to use the same word of the deceits of the Enemy." - Galadriel in The Fellowship of the Ring
I even invented a word to use as a general term to replace the word "magic" in discussion of Fantasy: "cobha" (a word borrowed from one of my old made-up languages), and recently I have been using or coining various terms to use in the various stories I invent: "sophology" and "royal philosophy" for one, "summoning" for another, and so on.
I disagree with celebrating Halloween simply because it is a pagan holiday. It seems ridiculous to me to celebrate a holiday because it means nothing and has no connection with its own past. If someone wants an innocent celebration, why pick a pagan holiday?
But as to reading dark subject matter as opposed to only reading things that make you happy, this I would say is simplistic as a principle, but innocent enough as a preference (though you would have to avoid much of the Bible and most of the best literature that exists in order to follow this preference completely). It is good to be sad about sad things, and to be repulsed by evil, and to face hard things with courage, and to realize the sufferings of the oppressed and those who fight for them: these all would fall in the general category of dark and in some cases not-family-friendly category.
A lot of horror and tragedy and romance media is poor quality: for the same reason that a lot of media specifically designated as "Christian" is poor quality: these categories often rely simply on being in that genre to get attention and to get away with poor quality. Shakespeare, however, I think is worth reading, at least for adults. Dracula is very good as well (it has a good ending, by the way).
Incidentally, I find ridiculous the idea of a "good vampire" that only drinks animals' blood.
