It would appear that the etymology translates to "grasping the required sound" or "grasping and needing the sound", though the latter doesn't make sense. "To grasp" makes sense, and "metal sound" could evoke meditation bells or other audial meditation aids that symbolize or inspire mental stability and clarity, but nge is hard to translate. It could be that nge is not indeed short for nge mo, and, as Rangjung Yeshé suggests, goes with ting to mean "clearly," which makes better sense but is not corroborated by other dictionaries.
The only time that Roerich renders ting nge 'dzin in the Sanskrit form of "samadhi" is when it is part of the title of a sutra or other text. In all other cases (except for names), he attempts to translate the term. Of greatest note is the dominance of "trance" and "concentration." The term sgom pa is typically translated as "meditation," and Roerich seems to translate ting nge 'dzin as such only when the meditation is specifically of the kind/stage/strategy that deals with equanimity or stabilization. The exception to this is the "Meditation on the Supreme Reality," which Roerich surely translates in such a manner because of its institutionalized quality. It evokes an established practice more than concentration or trance can, and thus Roerich generally uses these latter terms to describe ting nge 'dzin in more ad hoc, revelatory contexts. "Trance" is perhaps the most unusual choice given its half-conscious and subtle visionary connotations. The first usage, on page 3, makes perfect sense, but further usage is suspect, especially when interpretively modified with the "mystic" adjective of which Roerich is clearly very fond. Mysticism is justified in certain of these contexts (e.g. the sudden perception of non-substantiality), but the others might have benefited from a more formal technical translation that adheres to the system of practices prescribed.
Below is a sample of the many instances of ting nge 'dzin used throughout the Blue Annals.
Samadhi
"In order to produce in the minds (of others) the adhisamādhi-śīkṣa (lhag pa ting nge ‘dzin gyi bslab pa)…" (165)
"The so called Mahāmudrā agrees in meaning with the Samādhirāja sūtra" (268)
Trance
"Maudgalyāyana then entered a trance in which he was made to see…" (3)
"there were 500 who attained a concentrated trance" (292)
"his mind was suddenly plunged into the mystic trance in which one perceives the non substantiality" (298, 301)
"There, after the lapse of one year, an excellent mystic trance was born in him" (300)
Name
"myang teng nge 'dzin bzang po" (107)
Concentration
"the place of the Vajropama concentration of all the Sugatas of the three times" (140)
"He developed his mind concentration" (147)
"He conveyed numerous beams of Juniper wood from phug chung rong by the power of his concentrated mind, and said: "Here is the timber for the construction of the chapel by you, lady!" (179)
"At first when he was practising meditation, the noise of doors being opened and closed, hindered his mind concentration" (290)
"Then by placing her hand on my head a proper concentration of the Mind was born in me" (850)
Meditation
"Meditation on the Ultimate Reality was abandoned" (204)
"he said that it was due to different methods of establishing equanimity in meditation" (336)