I'll give the manga a pass for this since it specifically shows them drinking BLACK canned coffee, y'know, the brand.
But... BLACK COFFEE JUST MEANS COFFEE WITH NO CREAMER OR MILK IN IT. Coffee with JUST sugar in it is still black. If for some bizarre god damn reason you wanted to ruin perfectly good coffee by adding molasses to it, it'd be even blacker black coffee yet it'd still have a sweetener in it.
STRAIGHT coffee is coffee with nothing added to it, at least when it comes to drip brew and non-espresso/cappuccino.
My brother, somehow, drinks straight coffee. For the life of me I don't see how. My coffee needs to have some sweetener added to it or I just can't enjoy it. Coffee benefits greatly from sugar/sweetener or, if you like it, cream. Just because you can drink it straight doesn't mean it's the best way to enjoy it. I can hand you 190 proof Everclear and you can drink it straight... and I'll call an ambulance for you. Just because you're enjoying it unaltered doesn't mean you're enjoying a superior drink. They put sugar packets and creamer next to the fucking coffee for a reason. Enjoy it at your own leisure and ignore anyone snorting at you adding what you wish to your coffee.
Oh, but like NTR, decaf is never okay. Show the coffee respect. Don't cuck yourself or the coffee.