If the block size is bigger, doesn't that offer protection from spamming the blockchain as it will be so much more expensive to do?
Block sizes are chosen by miners. Miner get to choose which transactions go in blocks, if they include lots they'll have bigger blocks. Its the blocksize limit that is being changed, which is the biggest size block a miner can make.
So why would you need a *blacklist system*, (if that is what it is?)
XT contains some other changes unrelated to the fork and one of those is the somewhat controversial Tor blacklist. The main concern with this patch is that it pulls down a list of Tor nodes from torproject.org each time it starts. torproject now have logs of every time an Xt node is started along with it's IP, which they could use to try and determine which nodes were online at particular times. They could use that kind of information to de-anonymize users by figuring out which nodes were online whenever the user makes a transaction, any nodes that are continuously online whenever the user makes a tx are the ones that are most likely to belong to them.
Though It's not known what torproject would do with this information, I doubt they would use it for evil, but it is still a risk and other Bitcoin wallets have removed stuff similar to this for this reason.