[Deathblob]

The other units don't even have to be sacred to screw up a self-bless. Here is a Dai-Oni trying to bless himself. He's not very smart, so instead of targeting himself (at x6 y15), he targets his non-sacred wolf in (x5 y16), and hopes he gets a spalsh bless. He doesn't. Here's the relevant -dd info:

deploycom Shingen at 6 15
com Shingen cast spell (favspell Blessing) (mayusegems 1)
best Blessing this far, 5 15 (112 pnts)
castspell: cnr261 spl754 (Blessing) vis0 x5 y15 spldmg1
blastsqr: unr7365 x5 y15 aoe1 dmg1 eff10 spc1086373888 as10217 al9
affectvic vic7367 hv0
hitunit 7365 7367 dmg1 spec1086373888 ba-1
blastsqr: unr7365 x4 y16 aoe1 dmg1 eff10 spc1086373888 as10217 al9
blastsqr: unr7365 x6 y16 aoe1 dmg1 eff10 spc1086373888 as10217 al9
blastsqr: unr7365 x5 y14 aoe1 dmg1 eff10 spc1086373888 as10217 al9
blastsqr: unr7365 x3 y17 aoe1 dmg1 eff10 spc1086373888 as10217 al9

It looks like there's a no-debug-output stage building the scores for spells, where Bless is targeted at various squares, and scored based on the outcome. Perhaps the Dai Oni was splashed with a bless during the trial run cast at (5,15). I can't tell if (6,15) was even tested, since there's not enough output.

If you have a mixed squad of sacreds (S) and vanilla units (V) like this:

SVS
VVV
SVS

Then it may be optimal to target the center vanilla square.

But when there's only a single sacred unit on the battlefield, it's completely buggy that the unit does not target his own square when casting Bless.