Blind Sage (BS) and Leviathan (L) require approximately the same number and type (Eldritch and Ascendant) of devotion points, and have similar procs and bonuses. In my opinion it appears that BS is (almost) always better than L, with few relevant exceptions.
Passive bonuses comparison:
+180% fire/light/cold dam vs +180% cold dam
+200% burn/elec/frost dam vs +100% frostburn dam
+10% crit dam
+20-34 cold dam
+45 frostburn over 3 sec
+20% fire/light/cold res vs +20% cold res, +20% pierce res
+25 DA vs +40 DA
+30% skill disrupt protection
+20% energy regen
Overall, BS provides equivalent cold damage but better DoTs and much better fire/lightning damage. While L provides valuable flat damage, BS provides OA and valuable crit damage.
Triggered Skill comparison:
Trigger: 100% chance on attack vs 30% chance on attack
Cooldown: 1.2 seconds vs 3 seconds
Radius: 3.8 mtr vs 3.5 mtr
Duration: 3 seconds vs 6 seconds
Other: 2 second stun after 3 seconds, energy burn vs 40% slower movement for 2 seconds
Damage: 400 elemental DPS + 555 elemental burst + 83 from energy burn vs 420 cold + 340 frostburn over 2 seconds
Realistic fight comparison
Lets put that into perspective of a fight which lasts 12 seconds, starting from the first time the skill is triggered, and assuming that both skills always trigger:
BS: 14480 elemental damage + up to 660 energy burn damage = 15140 damage
L: 8820 cold damage + 3910 frostburn = 12730 damage
So when both skills perfectly trigger on time, BS does substantially more damage even when ignoring energy burn. Furthermore, the damage from BS is actually mobile and seeks out targets. On top of that, Leviathan’s skill only has a 30% proc chance, while BS has a 100% proc chance. As such, L will have substantially lower damage especially in 1v1 fights.
In terms of passive bonuses, BS shines in providing much better DOT damage, as well as damage bonuses for all 3 elements instead of only cold. The +5% OA and +10% crit damage from L is arguably more valuable than the flat damage provided for by L. L provides fewer resistances, but does come with 15 more DA, +5% HP and +25 physique. Nice, but nothing spectacular. The +30% skill disruption protection from BS is a more rare defensive trait.
In terms of the procs, BS is superior to L in all regards. It procs more often, faster, does more damage, has a larger AoE, stuns instead of slows, deals 3 types of damage instead of 1, and seeks out targets.
Refuting the “cold build” exception:
Of course an exception is when you focus on cold damage but not on fire and lightning. There appear to be only two plausible scenarios in which this is true: a necromancer which deals cold+vitality(+aether) damage and a nightblade which deals cold+pierce damage.
However, I would like to argue that these exceptions (and possibly any other builds that you could come up with) would still likely benefit more (or equally) from BS than L. In both cases, we are necessarily working with the assumption that the build has a lot of Ascendant and Eldritch devotion points. These devotions predominantly provide total elemental bonuses over cold-only bonuses. It is certainly possible to chose your devotions as to only provide bonuses to cold and to no other elemental, this only appears plausible in a build which has another damage type as the main instead of cold. In that situation you are not likely to want to run L anyway, but run with a devotion of your main damage type(s).
In short, if you are interested in Leviathan, you are likely to benefit more from Blind Sage.