It will not move your hp to -1 when cast as an Immediate action
You have two options when casting Blaze of Glory:
- Using a Standard Action, you decide that you want to go unconscious in combat (for whatever stupid reason) to trigger the spell effect. This drops your HP to -1 and unconscious but will keep your character stable (you won't bleed unless you take additional damage);
- Using an Immediate Action when something drops you below 0 HP, which could mean the spell cannot be used to save you from death, as Immediate actions do not happen before the condition that triggered them (see this answer for a full explanation);
Either of those methods does not affect how the remaining of the spell works:
When you cast this spell, any good creature within range is healed for 1d6 points of damage per two caster levels. All evil creatures within range take the same amount as damage instead (a successful Will save halves this damage). In addition, all allies and enemies within range are affected as if by the prayer spell for 1 round per caster level.
*"Any good creature" and "all allies" includes the caster, if they are actually a Good creature for the first part, and always for the second part because you are always your own ally. It is possible for a neutral, or even evil, character to cast this spell and not be affected by the first part of the spell.
The first part may heal you back up, for 1d6 per two caster levels. So this is why you use this as a standard action, because you will be healed back up. But regardless, you will still drop unconscious and will be prone when happens, so be careful to not cast it while within range for attacks of opportunity or you could drop unconscious again. And you certainly shouldn't use it when you are above 10 hp, as the amount healed is not worth the loss of action and all the extra hp lost.
And for the second part, you are affected by an effect similar to the Prayer spell, which will grant bonuses to your allies and penalties to your enemies.