Thanks for the chapter!
You all are forgetting this isn't a battle to press on and save the human race, but an entertainment 'show'. Also, for reasons that'll likely be explained later, MC was restricted by his mentor not to stand out too much or do anything that'll cause him trouble. Hence MC doesn't cast it on humans and problem with casting it on monsters is that due to the effects it's hard to tell whether or not he did something much less what he did. If they bothered to talk to the monsters about the effects they'd find out how powerful and fearsome his ability is, but humans like to favor humans and believe what they can see so any monster interviews only show up on the monster channels. Also, which is Alvar's main problem, is that casting dark magic doesn't have any visible, entertaining effects like a wave of darkness or monster afflicted changes color and in LN he commented that trying to chant anything just irritated people for some reason. Personally I think he should combine his Black Magic with stage magic. Basically do tricks that look good but do nothing while casting the magic so he at least appears to be doing something and raise some entertainment value. Like smoke spurt out of his sleeve for a couple seconds while yelling out "Blind" at target pointing at while casting black magic to blind it.
Btw, wasn't mentioned here but along with it's lack of entertainment value and difficulty to determine if actually worked or not, one of the reasons Black Magic is looked down upon is it's believed to be a beginner step for Black Sorcery. Black Sorcery being the really evil spells used by the Demon Lord to suppress and kill humanity making it highly illegal to use much less know from the passed on fears. Although hero abilities are feared too if hero disfavored or not charismatic enough to keep an audience as being very powerful but without the stigma of having been used to try to eradicate humanity. Hence due to the stigma along with being hit or miss and weak effects by most of the users Black Magic users are very disfavored. Especially as higher the level of opponent the harder it is to get to work along with a weaker effect. For typical Black Mage casting blind on a goblin might have an 80-90% chance of success to completely blind the goblin for 10 minutes. However, casting it on a high level hero like Phoenix with magical resistance might have a 5-10% chance of blurring his vision for 5 seconds. Hence when fighting monster Black mages the usual reaction is to brute force through it. Also, typical Black Mages can only do one or two spells on one or two targets at a time and getting a penalty for using weapons or armor not related to Black Sorcery makes them very susceptible to attacks requiring someone else to protect them.