This term is the condensed form of bla ma chen po, meaning "great lama." Though it is impossible to say for certain, one can imagine that "great spirit" came to have significance of its own, however, less formal than its expanded definition.
bla chen is used in three very similar contexts throughout the Blue Annals, in order of dominance, as 1) a religious title prepended to a proper name, 2) a formalization of the title into a personal name, as in the case of bla chen po, and 3) a purely descriptive adjective extrapolated and generalized from definition #1 to refer to a category or group or unnamed individuals, though one could make the reverse argument that bla ma chen po was the original informal usage, which then led to its titularization. Based on the evidence in the Blue Annals, however, bla chen is used primarily in its first two forms, and only later in more general contexts to refer to groups or unnamed great lamas, suggesting that the term can be comfortably used in this more detached manner only after having been established in its primary forms. In this way, it may be more proper to consider the final definition a caveat to the first instead of breaking it off as its own entity, but sources outside the Blue Annals suggest the discreet third definition.
The bod rgya tshig mdzod chen mo lists the definition of bla chen as "bla ma che khag" or "greater kind of lama", a definition that implies the term's potential for usage as a title but doesn't explicitly require it. Indeed, another definition in the bod rgya, for rje btsun dam pa, reads, "sog po khal kha'i bla chen zhig…" (p. 910). Here, the use of the indefinite particle "zhig" makes it clear that rje btsun dam pa belongs to the bla chen category of lama, but uses the term as a descriptor instead of a title.
It is telling that Roerich chose not to translate bla chen into English in nearly all contexts, except when referring to the figure of bla chen po, whom he calls "the Great Lama" only once, in the opening title of the second chapter, and in a few isolated cases, such as on 209-210, when he calls 'brog mi "the great teacher". In all other cases he leaves the Tibetan alone, referring to "bla chen po" and "bla chen 'brog mi" and "the bla chen rin gzhon pa" and so on. This is a clear indication that Roerich typically considered the term a title or name only, but occasionally shaded into the third definition.
It should be noted that these designations are purely artificial, for the sake of capturing the subtleties of the term's usage, and almost certainly wouldn't be admitted by Tibetan authors, who would perhaps give the definition that the bod rgya gave.
Overall, the Blue Annals uses the word consistently to refer both to individuals who have bla chen as an adjunct name and to individuals of great esteem, either because of their scholarly achievements or general reknown.
Type 1: 9 total, 5 unique
"bla chen dgongs pa rab gsal" (34, 167), aka bla chen po (see below)
"bla chen po lha lde btsan" (68)
"bla chen po 'brog mi" (72, 204, 360)
"bla chen bsod dbang" (396, 710)
"bla chen rin gzhon pa" (695)
Type 2: 11 total, 5 unique
"Chronology of the later spread of the teaching, the Great Lama, and other events (bstan pa phyi dar gyi lo tshigs dang bla chen po la sogs pa’i skabs. Chengdu 89; Roerich 63)." (63)
"While he was thinking thus, several powerful asuras, known as the "Nine Brothers-dwarfs" (thi'u rang spun dgu), residing in the neighbourhood of Mount dan tig (ri dan tig) , perceived bla chen po and were filled with faith." (65)
Other bla chen po references (66, 67, 246, 649, 1062)
"From bla chen ('brog mi) he acquired…" and "a direct disciple of bla chen." (215)
"From bla chen pa he heard the remaining part…" (592)
"bla chen pa" in a list of names (1023)
"bla chen kun blo" (634)
Type 3: 4 total, 2 unique
"As stated above the great Teacher (bla chen) 'brog mi propagated the doctrine of study and meditation…" (209)
"Though the great teacher (bla chen, i.e. 'brog mi) had many disciples…" (210)
"the bla chen rin gzhon pa…" (588, 589)
"the bla chen pa (rin gzhon pa)…" (721)
The above are tricky because they ride the line between title and proper name, but are translated here to be descriptive.