The documented outline is firm enough. She was born Helena von Hahn at Yekaterinoslav in 1831, married Nikifor Blavatsky at seventeen and left him within months, travelled widely and unconventionally for a woman of her class and century, appeared in New York in 1873, and co-founded the Theosophical Society there in 1875 with Henry Steel Olcott and William Quan Judge. She moved the Society's work to India, wrote Isis Unveiled (1877) and The Secret Doctrine (1888), and died in London in 1891.
The rest is her own testimony, and it is worth setting out precisely because it is so often either swallowed or silently dropped.
She said she had been trained by a hidden brotherhood of Masters or Mahatmas, chiefly the adepts she called Morya and Koot Hoomi, and that she had spent a period in Tibet under their instruction, in her own accounts roughly 1868 to 1870, though she gave the story inconsistently and the frequently repeated "seven years in Tibet" conflates the journey with her wider claim of seven years' training. There is no independent evidence for any of it. No corroborating witness, no document, no record of the journey. Nicholas Goodrick-Clarke notes that the claims rest entirely on her own word. Some of her defenders have observed that her grasp of Mahayana ideas was better than a purely literary acquaintance would predict; that is an argument from plausibility, not evidence of the journey.
It was disputed in her lifetime and by people who went to look.