If Blackrock etf is actually accepted they have unilateral decision making powers as to which fork is the reference chain for pricing. In the general public eye and with a major assist from msm fin-news media outlets, its likely whatever chain they choose will be regarded as the "real" bitcoin.
Doesn't change the fact that bitcoin is bitcoin and consensus & game theory will ultimately rule all eventually, but in the meantime it'll open the door for some major major fuckery.