If Black Blood of Y'Shaarj is anything like Purified Bindings of Immerseus, then the int proc won't proc while you're in a healer spec
I've tested the Black Blood trinket in PTR a few minutes ago on a target dummy and can confirm the trinket definitely procs from atonement.
Testing this trinket was fun and presents possibilities for example popping Halo near the end of the buff and you have 23.5k int + approx 10% extra crit from int. I saw some nice numbers. With our mysticism passive, raid 5% int stat buff, and 10% raid sp buff, all up an extra 28.5k spell power during that last second of the trinket buff.
One thing I did notice during testing, it appears Halo doesn't snapshot the int proc at cast. If you cast Halo just before the trinket buff expires, say during the 10th sec, and the trinket buff drops whilst the Halo field is expanding, the healing/dps numbers seem not to include the proc, so if you are going to cast Halo, do it around the 7s mark.
I would love this trinket on my main to play with, but our raid group has to get Garrosh down first. We are at 11/12 :(
I think Disc priests should consider this trinket as an alternate throughput trinket, it definitely has potential to pull big numbers but big numbers will mean naught if it only results in big overheals.
My thoughts only