A proper Black Core card remains 100% opaque
The limit to Johann's ability--that you can only do it once each turn, is important, because that means you can do it on yours AND your opponent's turns
Shoutout to the MTG Judges Chat Discord for their availability and time
Commander is the most fun when everyone involved is aligned on what type of game is going to happen, and discussing this should be the first thing you do before playing with a group
Better yet, casting two or more non-creature spells (which you'll likely be doing already anyway) gives Lyse Hext double strike, meaning you'll be dealing double damage with any attacks on top of whatever effects come from the non-creature spells you've been casting (which, again, are cheaper thanks to Lyse Hext's signature ability)
Let's take a look at the building blocks for brewing Zombie decks in a new Standard format