What if there was one or many massive global shared ledgers all these institutions ran nodes for? You could transfer ownership of a security between someone in China and Europe instantly.
So it's better to ask companies to invest in implementing "blockchain" within their network instead say upgrading their databases or their infrastructure to the current technology level? Then they could simple have API/Middleware calls between systems which can do this even more efficiently.
If there's a particular group responsible for paying out the security, then it makes more sense to me for that group to just host a ledger on their site and officially endorse all transactions (which can use the same crypto that blockchains use). This would be instant and transparent.
I don't understand how security trading works, but if there is truly no trusted authority responsible for either paying out the security or enforcing that it gets paid, then I guess I see the possibility for using a blockchain (but probably you also would want cryptocurrency...).