Whether you use a blacklist or a whitelist... I think you must create the one which is smallest and/or with the least impact. I think a blacklist would be smaller, if you allow no words smaller than 4 characters to be used anyway.

You could add tagging to a response. So for example in your example the response is tagged with 'server' and 'down'. It would be possible to create a search on the tags.

I created a simple chat bot a year or so ago now. The way i went about it was to store keywords in the database that relate to a specific question, or desired output. (This is compiled with a very simple form)

To search the database I break the input down into an array of single words and use a group of functions to search for the entries whos keywords are all in the input array. I can if i want have a selection of different outputs for any set of keywords.

-- I did say it was simple! --

This is a very basic method but it does work. and allows an automated response from a "question".

A lack of spare time has prevented me from developing my chat bot much, but I would love to have a system that not just replies but can ask related questions back and be a little more interactive than just a dead end response, I have some ideas for that but its a huge task, and somewhat complex! If anyone has any ideas on how to structure this I would like to hear from you.