Enchant Weapon - Black Magic: now cause your harmful spells to sometimes increase haste rating by 250.
I'm going to use Graylo's numbers and put haste rating (past the soft cap) at approximately .8 spell power. Then the proc is worth 200 SP over 10 seconds and would have to have 63/200 = 31.5% uptime to equal a spell power enchant (about 23% uptime if you were below 150 haste, a pretty unlikely scenario).
I managed to test this for about 15 minutes before connection issues forced me off the PTR. I'll update this once I have better data, but some initial observations:
I never saw it proc twice within ~30 seconds (two Eclipse durations). It also never refreshed itself. So that strongly suggests there is still an internal cooldown, probably of 45 seconds.
I don't know the proc chance for sure, but it doesn't appear to be even as high as 50%. I did four tests where I waited 1 minute between procs, and the number of casts to proc were: 2, 5, 11, 6.
Using the ICD of 45 seconds and estimating the proc rate at 10% (based on similar spell effects), you should expect roughly 1 proc per minute, or a 16.66% uptime. That puts Black Magic at roughly 50% the dps of a 63 spell power enchant, so it's definitely not worth it.