Edit: Not to mention, immutability means human errors cannot be recovered from - a wrongly worded contract cannot be restored.
Negotiations would be done by the "servicer" of the loan, who gets to charge a fee for this. There is also the "originator" of the loan (think the website where the person taking out the loan clicked on the affiliate link, but in meatspace). The "owner" of the loan is generally considered to be the person who currently puts up the (remaining) principal.
Each of these roles may or may not change hands as a result of a transaction, but generally only the risk role changes.
There is a large business "originating" loans, and over time during the credit cycle the business becomes originating ever more risky loans until the whole thing blows up and we get into panic, tighten lending standards (overtighten, generally), and go back into the business of loosening up.
I can't speak for the blockchain stuff but typically in financial software data is treated as immutable as well and errors are accommodated for by adding an additional correction record. I assume blockchain would enforce this practice, but then again, I don't know how it would distribute the amended contract.
But can't things like name of the person taking the loan can be changed? Obviously there is still a journaling entry recorded to track who and what changes were done. This is specially important because wrong names can and do affect credit ratings etc.
Nailing down this stuff is where you really appreciate a good architect.
Often, the people keeping the records of who owns what have become extremely powerful - it is my understanding that this technology has the potential to change that dynamic.
Go to the source code. This stuff is open-source and well documented. Once you start reading the code this stuff makes sense. Write your own blockchain. In fact it's not that hard and it'll make the basic concepts really sink in.
The only use case I know, beyond proof of ownership of coins/tokens (and, again, how can that be applied to a real world problem), is proof of document state at a certain point in time (via storing document hashes in the blockchain) which clearly has some uses in law for example, but apart from that I am stumped.