- Everything regarding rolls to hit from my first comment still applies.
- The Hellblaster weapon rules harm the user of the weapon, not the target.
- There are weapons that always hit currently in the rules; they follow as described in my first comment.
- The Deathwatch Frag Cannon is an example of a weapon that changes from one attack to the next.
- If a psyker power is created that will always hit it will operate similarly.
This is too long for a comment and if I am still misunderstanding your question let me know and I will edit the answer until we figure it out!
First let me say that if you do not have your head wrapped around my first comment that is important; if you do forgive me and skip this section
So from my first comment, during play you always roll to hit your target unless the weapon or attack specifically says otherwise. So once you have chosen your target you roll to see if you hit that target. If your roll is equal or greater than your model's ballistic skill then you have successfully hit your target. If not, then you miss. If you roll a one you always miss, regardless of any modifiers that may apply during that attack. i.e. Let's say you are attacking a target with a leader character nearby that has some ability that grants minus one to any model's ballistic skill that is within 6". Your skill is 2 normally and that took your skill from 2 down to 1. You roll to hit your target and get a one. While the modifier would grant you a successful strike, the fact that you rolled a 1, means that you have missed your target.
Second, I think you may be misreading the rules. Regarding the Hellblaster weapon the ability states as follows:
Each time a model fires a plasma incinerator, it can supercharge the weapon before firing. If it does so, increase the strength and damage of the weapon by one (1) for this turn. If you roll any hit rolls of one (1) when firing a supercharged plasma incinerator, the firing model is slain as the weapon explodes.
This states that you can choose to overcharge your weapon making it more damaging to the targets for the duration of one turn. If you choose to do so you put yourself at risk of the weapon exploding in your hands and killing yourself. On a roll of one (where you always miss regardless of modifiers) you have not only missed your target, but the model that fired the weapon has blown themselves up as well. That model is dead.
Towards what I believe your real question is, there are weapons/ attacks that DO always hit and you do not need to roll for hits because of the special ability. One example of this is the Rubric Marine's Warpflamer and Warpflame Pistol. These weapons have the same ability:
This weapon automatically hits its target
In this case despite the ballistic skill of 3+ you do not even have to roll to hit, because the weapon always hits its intended target. Theoretically, in this case, if you did roll you would roll a 3 every time. In keeping with my first comment, in this case you do not roll to hit when using one of those weapons, you have already hit your target choosing that attack profile and now need to roll to wound. In this case as well a target's ability to force you to re-roll to hit does no good, because you always hit. The only thing that can save them now is the wound roll or their save roll.
Similar to my comment the Deathwatch Frag Cannon is a weapon that has the ability to activate its always hit ability when attacking. If you use the 'Shell' profile and your target is within half its maximum range you get added benefits. If however you use the 'Frag Round' profile the weapon has the ability that it always hits. In this case you can choose which profile to use and its ability to always hit could change from one attack to the next.
If a Psyker ability (or any other weapon/ ability) is created that always hits its target it will operate similarly to these.
I do not know of any Psyker abilities of the top of my head that always hit, but it does not seem to be a likely circumstance. If anyone had the ability to always hit I would have assumed either the Magnus or maybe the Grey Knights to have such an ability. All of their available psychic attacks and special abilities say "If manifested" so even a Daemon Primarch and Grey Knights miss with their Psyker abilities.
This does seem interesting to me however, because it would mean that even Magnus the Red is subject to the Perils of the Warp rule; where if you roll snake eyes (double one) or midnight (double six) then you are attacked by warp manifestations... I would think Magnus would be exempt from that at least, but nope... sucks to be a Psyker!
If I have not gotten your question, please let me know and I will adjust my answer.