By Jess Jones American private equity giant Blackstone has reportedly been finalising a revised proposal to buy Hipgnosis Songs Fund (HSF) over the weekend. Blackstone is gearing up to submit a bigger bid for the London-listed fund that owns the rights to songs by Justin Bieber and Shakira as soon as Monday, according to Sky News. Blackstone’s offer will surpass the $1.25-per-share bid endorsed by HSF’s board last Thursday, which came from Concord Music, backed by Apollo Global Management, sources told Sky News. Blackstone and HSF both declined to comment. Concord’s latest offer is $1.25 (99p)…