I don't think the steem blockchain can be brought down, but it can be slowed down. A DDOS is basically supposed to slow down websites by adding load to the servers. The servers then shut down automatically or manually because of the over load.
The blockchain itself, can be similarly over loaded. Remember what happened to Bitcoin a few months ago? Thousands of unconfirmed transactions, high fees etc. STEEM can face the same thing but is better prepared because of the role of witnesses.
Coming to a consensus is much easier on STEEM, if the block sizes have to increase for the network to run smoothly, witnesses can do so and an average of their declared sizes will be used. That's how witnesses had solved the bandwidth issue on the steem blockchain two months ago.