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I noticed when Boruto opens his eye it starts out dark blue like normal but without the black iris then the light covers it. The Byukugan would always be permanent and the tensigan is always active when it became deactive it just became a regular bykugan.
Its probably due to Borutos senju dna (Hagoromos descendants through the uzumaki) mixing with Hamuras descendants (The hyuuga) creating a new eye, a new dojutsu altogether.