So to make my life a little easier, and to enable other people to easily add formatting and links to their comments, I've developed a script that provides buttons for bold, italic and for adding links. Unfortunately I couldn't find a way of adding something to a blogs template to make this work, rather I've developed a Greasemonkey script that, once installed in your browser, will add this functionality automatically to every Blogger hosted blog you visit.
If you are using Firefox then you will need to have the Greasemonkey extension installed. If you use Chrome then (due to an annoying change Google have recently made) you will need to install Tampermonkey, and for Safari you will need to install NinjaKit. The script also works in Chromium (the open source version of Chrome without the Google branding) without requiring you to install an extension first.
Once you have your browser ready you can simply click this link to install my script. If you are using Tampermonkey then, when asked, click OK to install into Tampermonkey rather than Cancel to install natively in Chrome.
Once installed whenever you go to add a comment on a Blogger hosted blog you should find links for bold, italic and to add a link just above the comment box. Hopefully there shouldn't be any bugs as a) it's a fairly simple script and b) thanks to Adrian it's actually undergone some user testing, but if you have any problems or any suggestions for improvements then leave me a comment and I'll see what I can do.
Updated 14th July 2013: Added the link to NinjaKit for getting it to work in Safari.