it it possible or even practical to identify a bitcoin as having been a "direct descendant" of a coin involved in a given transaction and/or as a coin that has been "co-mingled" with such a coin?
Definitely. That is easy to do. However since each transaction can have multiple inputs and outputs, the set of descendants is likely to grow over time, until eventually most bitcoins are descendants of that transaction.
it may make it practical to for major players and for that matter anyone who uses BC to "locally blacklist" seized bitcoins.
If there isn't any consensus in the "community", then such a blacklist is unlikely to have any effect.
If some miners decide to blacklist transactions involving certain coins, then other miners are just going to pick them up. If only a minority of miners are in on the blacklisting, then this is going to cause a fork in the blockchain. Other miners have to decide, which fork they are going to bet their resources on. If there isn't consensus on what to blacklist, there could be so many forks blacklisting different subsets, that each fork is going to become irrelevant leaving only the chain with no blacklisting as viable.
Even if you could manage to get a majority of miners to agree on exactly what should be blacklisted, it is of questionable value to the miners to attempt blacklisting. It could be seen as introducing a dangerous precedence for introducing blacklists. This would introduce a new and even more unpredictable danger to anybody owning bitcoins.
Traders could decide to blacklist certain bitcoins. This would mean you would refuse to accept blacklisted coins. But if you are selling goods for bitcoins, then you'd have to announce in advance, which coins you consider blacklisted, otherwise you'd have disputes where the buyer of goods says they have paid, but seller of goods says the received bitcoins are no-good. And as receiver of bitcoins you'd also have to decide how diluted the blacklisted bitcoins would have to be, before you'd accept them. And in all, there'd have to be consensus about both the set of blacklisted bitcoins and the dilution threshold. Otherwise nobody will know, if the bitcoins they are accepting are good or not, and without such knowledge blacklisting wouldn't have the intended effect, instead you'd just be rejecting arbitrary payments, you might as well flip a coin and say no-thanks to a certain payment.
I think the only consensus that has a real chance of being reached is that bitcoins are not blacklisted.