They don’t have to be gargantuan (except in terms of mass) - the low end of stellar mass black holes can have radii as small as 6 km.
Many black holes have a bright accretion disk outside their event horizon, so visibility for those is not an issue.
But it’s certainly possible to have maximally scary black holes that have no accretion disk and are small enough so as not to cause significant lensing. You could fly your spaceship straight into one of those without even realizing, until you start turning to spaghetti.
Yeah I remembered the accretion disks immediately after posting and edited the comment.
> it’s certainly possible to have maximally scary black holes that have no accretion disk and are small enough so as not to cause significant lensing
Now that's a proper cosmic horror.
The event horizon might have a circumference as small as 12*Pi km but as I understand it, the radius is infinite. ;)