Sometimes transaction is not reported, not treated as a capital transaction.
Blackstone - if you read Blackstone literature i think they discuss the tax treatment - they may have already gotten an IRS opinion or ruling on the transaction.
For example My first take on Blackstone converting to C; hm, PTP suspended losses, will increase the basis of Blackstone Stock received in exchange for Pship interest.
I have not confirmed that or settled that in my mind, but that is an example. Of course investment companies don't have any history on the tax side, so they will refer to cost basis as being cost of buying MLP units, exclusive of adjustments for pship income/deductions passed to partner