Other answers are in error as to the first appearance of the mind blank spell in D&D. The first appearance was in Supplement-I for the original Dungeons & Dragons game, the Greyhawk booklet by Gygax & Kuntz (1975):
Mind Blank: By casting this spell on any person the magic-user prevents any form of detection by ESP, Clairvoyance. Clairaudience,
Crystal Ball gazing (including any other form of skrying) Wishing
Commune, or Contact Higher Plane. Duration: 1 game day. Range: 1".
This description was further expanded in the 1E Advanced D&D Player's Handbook (1978), before also being included in Oriental Adventures (1985). In particular, it did not originate in an asian-themed book.
Note that the first item of concern in the description is ESP (extra-sensory perception), which is fundamentally a mind-reading spell. Furthermore in original D&D, both clairvoyance and clairaudience were expansions on the ESP spell (e.g., from Volume 1 in 1974, "Clairvoyance: Same as ESP spell except the spell user can visualize rather than merely pick up thoughts.").
So the kernel of the spell's intent is to protect from ESP-type mind-reading, by making the mind effectively not there for detection. The other related divination-finding spells are then tacked on or expanded at a later date.
As an English idiom, having a "blank mind" can refer to not having any thoughts, possibly through meditation or being depressed. Looking at Google Ngram, we see that "blank mind" has had close to constant usage from around the years 1860 to 2000. Meanwhile, the phrase "mind blank" lagged behind until after 1980, when it spiked upward in usage -- I'd hypothesize that this represents usage inspired by D&D itself.
In the modern era, the phrase "mind blank" has become a recognized term used in academic psychology papers, for example:
- Ward, A. F., & Wegner, D. M. (2013). Mind-blanking: When the mind
goes away. Frontiers of Psychology, 4, 650. doi:10.3389/fpsyg.2013.00650
- Kawagoe et al. (2019). The neural correlates of "mind blanking":
When the mind goes away. Human Brain Mapping, 40(17), 4934-4940. doi:10.1002/hbm.24748
And the latter article states:
Several previous works indicate that people often spontaneously
describe their minds as being “blank” in a free-response format
(McCormick, Rosenthal, Miller, & Maguire, 2018; Schooler, Reichle, &
Halpern, 2004; Watts & Sharrock, 1985).
Compare to other related English idioms like "go blank", "draw a blank", or "blank on (something)", each of which indicate forgetfulness or being unable to find something.