Bloodlines improve all abilities dependent on class lv, and this applies equally to all classes/prcs you have.
Let us say that you are currently a warlock/hellfire warlock3 with a major bloodline (any one). The 3 bloodline lvs (at lvs 3, 6, 9) apply to both your warlock and hellfire warlock lvs. This means that you are treated as having the equivalent of 6 lvs in hellfire warlock (because the ability explicitly words it as being keyed off HW levels), so your hellfire blast deals an extra +12d6 damage.
Lets say you are a warlock9/hellfire warlock3. You get a bloodline lv at lv3, 6 and 9. So at lv12, you use EB as a warlock14 (9 from warlock, 2 from HW, 3 from bloodline) for 7d6, and 10d6 from hellfire warlock.
Obviously, you will want to stagger your HW levels so that they will not get "eaten up" by the bloodline drawback (ie: take them at lv10, 11 and 13).
