#1. Blog commenting is Not considered spam. However, so many online users comment for the sole purpose of obtaining backlinks, instead of genuinely contributing to a blog post. This is widely recognized as "comment spam." Because of this, blog authors employ "no follow" on their blog comments AND Google recognizes comment spam as a problem - so they do Not use the links as ranking factors. They might see them, but they mostly disregard them.
#2. Google does Not use Meta Keywords in web ranking (as per Google Engineer Matt Cutts ). The Meta Description, if crafted well, could be used as part of the snippet that is returned on Google's Search Engine Result Pages. It is my understanding that both Meta Keywords and Descriptions do not have very much ranking power.