I'm entering a contest where I have to design an enchantment care with CMC 3 or more... and it has to be something Spike would want. I am, unfortunately, terrible at knowing what Spike wants.

Heh. Spike wants to show off how smart he is. If the contest is run correctly, it shouldn't cater to under-powered threats, but give bonus points to cards that can be minutely optimized, require skill to play, and tests Spike's thinking power against it's opponents. It doesn't hurt to have an aggressive cost, either. For those reasons, Fact or Fiction is often trumpeted as the Spikiest card in existence. If you don't like that card, you just don't have any Spike in you.

So, evaluating this card by Spike's harsh standards, I would say: "Okay, cute. It gives me a bonus that I will always use, since I won't play with black. That being the case, why didn't it give me a bonus that wouldn't be used in Red or Blue either, since I'm clearly not playing those colors (I'm assuming that Spike is going to forget that this card will appear in a Hybrid set). Also, if you really want to test my skill, though, why not, instead of giving a bonus to non-black creatures, give the bonus to blue, black and red creatures? That way, I've got to think to make this spell operate... and suddenly, I'm made aware that this card is probably in a Hybrid set. I could pack this in a deck full of White/Blue Hybrid creatures, or Red/Green Hybrid creatures and feel smart about it.

As for the graveyard shenanigans, that's nice. Personally, though, I think I'm going to forget to activate this on occasion, and that will make me feel dumb... and I HATE feeling dumb. Maybe this would be best if it activated for a bonus, then exiled itself? Then I could get optimum value out of my spell, find sneaky ways to discard it to get a quick effect, but not have to keep thinking about the damn enchantment in my graveyard, when I could be thinking about more important interactions in my deck. (Also, you probably don't have to bother with 'non-color' in the graveyard. I think you got the point across with the first ability)."

Yeah, I really don't like Fact or Fiction... but I already knew I wasn't a Spike. :)
Anyway, to me, it sounds like you're bleeding Johnny into your Spike a little by asking for a test of skill. I thought Spike just wanted to win, no matter what, and would therefore always play the "best" cards and decks?

Johnny wishes to express himself. He often does this by building decks that bring out his uniqueness. He does this by creating combinations of cards that he finds, and only he can harness, to do something special. If someone else makes a combo deck using certain cards, Johnny can appreciate it, but is no longer very interested in doing it himself. Spike, on the other hand, has no qualms of picking up where Johnny left off, taking Johnny's ideas and 'tweaking' them for maximum advantage.

So, yes, 'blue, black and red creatures get the bonus' could be Johnny. It can also be Spike. It is creative to find a way to combine that one card with a bunch of hybrid creatures, but it isn't that creative. Johnny would be bored after thinking about it for a little while. It would probably hit a deck that flirted around with this bonus with a bunch of other cards, and possibly included a completely different combo in the process. Spike would make a deck that maximized the bonus on this card, playing this enchantment, good creatures and good spells. Very straight forward, and winning.

Johnny, by the way, doesn't enjoy tests of skill. That's boring. Johnny is much more interested in creating something. If Johnny loses 9 games, then goes infinite in game 10, he's 'won'. Sometimes 'winning' isn't even defined by 'winning the game' for Johnny. If Johnny makes a theme deck where each creature represents a character from 'The Wizards of Oz' and he made them all 'follow the yellow brick road' (whatever that means), then Johnny has 'won'. Fact or Fiction, for example, is antithesis to this idea. It asks Johnny to make decisions that he's uncomfortable making, even if it makes it more likely for his idea to work. That's a bit like asking an artist "What part of your painting is the most important?" That sort of mind-think is what drives Spike, and drives Johnny nuts.

I disagree that Spike would say "Personally, though, I think I'm going to forget to activate this on occasion, and that will make me feel dumb... and I HATE feeling dumb". Spike likes having the opportunity to prove himself; Spike likes cards that give people the opportunity to forget to use them just so that Spike can feel he's proved himself when he remembered.