I came to this page trying to find how people were reading this myself and I think I understand the wording now that I have seen a few other people's ideas. I think its less of a command and more of an f(x) function.
They did not feel the need to make a whole chapter of items and feats JUST for the bloodrager and since he is a meeting of the Berserker and Sorcerer, they had to make them able to use the existing feats, items, and abilities. Think in terms of Min/Max (not saying I approve but it makes sense when you do). ANY combo of classes and abilities is possible, they want people to play their own creations, so you want things to balance and flow. Enter OTHER sources of DR.
Dont look at it as it was said earlier (Dr=0+1, DR=0+1, ect.) but as DR = (x)+(y)+(z), ect. The "Effective Damage Reduction" listed by this ablilty is talking about the DR granted by the Spelleater feature. In any other case, you LOSE Damage Reduction to gain Blood of Life, but it makes sure you know WHEN you gain it from any other source (like feats that tend to require you to HAVE DR to raise it; another example of why this line exists) you basically HAVE a DR 0. So the (x) is your DR gained from Blood of Life (effective DR from its point of view) and any other source is ADDED to the effective DR. So it goes to DR = (Spelleater or 0) + (other class feature) + (feats) ect. Each time you gain a new source of DR, you plug this new value into the ever growing equation.
The oddly worded line is a detailed way of saying "you lose the Damage Reduction ability from levels 7, 10, 13, 16, and 19 but for any future reference you have a base DR of 0." Because a DR/- is non-existant but a DR/0 means you HAVE DR and can therefor take steps to RAISE it, at the expense of a feat that would enhance something else or another class feature at the expense of going to level 20 as a pure bloodrager Spelleater.
This is, however, my understanding and can be as flawed as any other, but I do hope this makes as much sense to someone else as it does to me.