To be blank - or to call someone a blank - has a complex series of associations. Among the percentage of the population who can use magic without the assistance of tools, it's often thrown around as a slang term where you might expect to hear "muggle" or "squib" in the Potterverse.
In the oldest manuscripts, to call someone a clean soul, an unwritten or erased scroll or palimpsest, or, yes, a blank fate, meant that they had no echoes of past lives, obligations, failures or patterns to play out. In many esoteric and mystery traditions this is something to aspire to - unless you have not incarnated before, to have a blank fate means you have paid out all of your debts and scraped the tally clean. On an Enlightenment path, it precedes being subsumed into the Light itself, assuming you can make it through your blank life without incurring any new echoes or obligations. The ones recognized as blank often spend their lives in quiet temples or hermitage in an attempt to avoid attachment, training in the use of magic but choosing to forgo it except in defense or absorption.
It is this last trait that led to the misunderstanding in modern magical practice that referring to someone as blank means they are magically blind or even act as a null. Acting as a null was a highly trained skill for the traditional blank fated person, and is also a natural trait of those drawn to the role of the gaja, and in neither case is it a sign of someone lacking in magical skill or talent. Despite the cutting snark of many a magic-user, there is no such thing as being so bad at magic that you suck it out of other people.
Many of these esoteric paths have been lost over the years as one religion eclipsed another in popularity or as one government or another ferreted out those who resisted the wisdom of this or that emperor or king. Some documents remain in dead languages, and a few temples maintain their quiet ways.
Around the time the Emperor of Shenzhou fled to Mei Guo, there was a resurgence of interest in ancient techniques and a small handful of blooded scholars and adepts did their best to translate existing manuscripts into modern languages and techniques. This fashion for the wisdom of the ancients set two relevant sets of ripples into motion, one the attempts to standardize magical techniques and apply them to technology the way the medical knowledge of herbs and elements had been standardized and applied to biology and health care, the other the spread of magical practice by individuals and small groups of those with blood talent who lacked access to traditional teachers. The scientists had no trouble applying their own vocabulary and understanding to what they were looking at, while the eclectics were left to patch together the language and techniques of translations of varying quality. An early book building on those translations with original techniques began the modern use of blank when the author wrote of appealing to one's past lives and echoes to teach magic. Someone who doesn't have echoes to learn from is, yes, blank - but this is where the idea that blankness means non-magical comes from.
Figuring out how "blank" is used took some time, as I had to work backwards from the way various people used it in casual conversation, as well as how I was prompted to use it in metacommentary. This is very common in fictional recon, where language carries the weight of history but we don't necessarily have access to that history or etymology. As you would learn any slang, the trick is to observe the usage as much as possible and pin down the commonalities.
The technique referred to above, where one calls on one's past lives or echoes to teach them, is actually one I've used. It's basically the metaphysical equivalent of someone standing behind you as you swing a golf club, controlling your movement so you can see what it's meant to feel like. It is a very useful trick for learning things quickly, but requires you to put a lot of trust in the one teaching you. It's tempting to believe they must have your best interest at heart because they are you, but to be able to use the technique, they must have will, and if they have will, there's no telling what their goals are.