There are frequent references to black magic in Henry IV Part 1, so let's think about some specific moments where the supernatural comes into play.
King Henry refers to the Welsh leader as "that great magician, damn'd Glendower" (1.1.3) and when Westmoreland describes how 1,000 English soldiers were "butchered" and later mutilated by the Welsh, he refers to the man as "irregular and wild" (1.1.2). Such comments establish Glendower and Wales as mysterious, foreign, and dangerous. Later, Glendower goes to great lengths to assert his mysterious power by claiming he has conjured the devil to help him defeat King Henry's forces (3.1.7) and, at his birth or, "nativity / the front of heaven was full of fiery shapes / of burning cressets" (3.1.2). Sounds kind of serious…but only for a moment. Hotspur's reaction to this turns the whole episode into a joke. Check out what Hotspur says after Glendower repeats his claim that at his birth "the heavens were all on fire, the earth did tremble":
oft the teeming earth
Is with a kind of colic pinch'd and vex'd
By the imprisoning of unruly wind (3.1.5)
Hotspur demeans Glendower by comparing the man's birth to the release of uncomfortable gas, deflating, as it were, Glendower's serious attempts to shroud himself in mystery. This seems to also deflate the seriousness of the rebels' enterprise. In other words, it's not only impossible to take Glendower's claims about the supernatural seriously, it's also seems a bit hard (here anyway) to take the rebellion seriously. (Come on. The rebels are insulting each other with fart jokes, for Pete's sake.)
There are other references to black magic in the play, but they too are aligned with comic relief. When Falstaff convenes with his thieving crew at Gads Hill just before the double robbery, he curses Poins for hiding his horse.
[…] I have
forsworn his company hourly any time this two and
twenty years, and yet I am bewitched with the
rogue's company. If the rascal hath not given me
medicines to make me love him, I'll be hanged; it
could not be else: I have drunk medicines. Poins! (2.2.3)
Here, it seems we're not supposed to take seriously Falstaff's claims that Poins has "bewitched" or given him "medicines to make [Falstaff] love him." This is a highly comedic moment and Falstaff is making fun of himself for feeling such affection for a rascal like Poins. This also seems to make light of what might otherwise be considered a serious situation as the thieves prepare to rob the king's exchequer (treasury). The teasing and light pranking make the whole thing seem a bit harmless, wouldn't you say?