I am going to be in boulder for one evening and have to choose a restaurant , I am having trouble deciding between black cat bistro and the kitchen. Any thoughts? Also which one is has a more relaxed dress code.
Boulder is a relaxed town, neither has any sort of dress code. I think you'd be comfortable in jeans at either place, though maybe the kitchen is a little more relaxed.
Either restaurant is a good choice. I've only eaten at Black Cat once, and it felt a little "forced" to me, trying so hard to be farm to table that some things made it to my plate that really didn't belong. Still, the food was good. I really like the kitchen and have eaten there a number of times, it may be a little more affordable, if that is a consideration.
I agree with LurkerDan about Boulder being a relaxed town, and most restaurants do not have dress codes beyond no biking shorts/tank tops at dinner, and you should wear shoes! I have been to The Black Cat many times and have visited both their owned gardens and animal ranch on several occasions. Although I like The Black Cat, if you have only one night in Boulder there are other restaurants that are more interesting, and The Black Cat is better in summer than in winter for the simple reason that local, fresh produce is much more widely available and the variety much broader.
I don't care for The Kitchen, but largely for the same reason LurkerDan cites for The Black Cat - both restaurants feel very "forced" to me, they try too hard to be "perfect Boulder" in their character and in the end the result is not authentic - note that both bend over backward to name the source of every key ingredient so prove the "local" origin of it, I'm surprised that their menus don't cite the source of the glass of drinking water that they put on the table in front of you is the Arapahoe Glacier (Boulder is the only city in the country with its own municipal glacier!)!
You did not say how you narrowed your choice to The Kitchen and The Black Cat, nor did you mention any particular type of cuisine you are interested in, but you might want to consider Frasca, hands-down Boulder's finest dining experience - but it's pricy, reservations can be hard to get, and it's Italian, though you will never have had Italian like you will get there anywhere else, nor are you likely to ever find it elsewhere. One of the most underrated restaurants in town is Brasserie 1010 on Walnut Street. The menu is mixed with a heavy undertone of Continental, but with mostly locally sourced ingredients and excellent wines. 1010 is excellent for lunch, too.
For sheer drama Flagstaff House must be mentioned. It was Boulder's best restaurant until Frasca appeared. While pricy, perched half way up Flagstaff Mountain the views over the city at night are gorgeous! A couple of others worth mentioning: one doesn't think of fish when considering Boulder, but Jax Fish House can match or exceed any piscine experience you've ever had elsewhere; Mediterranean ("The Med") may be the most popular restaurant in town as well as what many locals consider the best - the cuisine is drawn from all countries that surround the Mediterranean Sea, and their selection of "small plates" (think tapas) is the best in the State.
I agree with some of keninboulder's recs. Frasca and Flagstaff are certainly top notch, though neither is a "relaxed atmosphere" so may not fit your bill. Jax Fish House is also quite good (and relaxed). 1010 is also a good place, but is pretty straightforward casual French. I like it a lot, but for one meal I'd choose the Kitchen (or the ones listed above in this post) before choosing it. The Med, well, I like it. It's so popular because it's a fun and affordable place. Great for happy hour, great for festive occasions. But the food is not nearly in the same class. It's decent, sometimes quite good, sometimes quite mediocre. Nothing really special and nothing I would tell someone looking for "great food", and considering the Kitchen and Black Cat, to pick over those places. Just one man's opinion. Have fun!