Part of having a cohesive blog design is to choose one or two fonts and two to four colors and stick with them on your site. The best looking sites are minimalist in design. A site that has too many different types of fonts and colors on it just looks unprofessional.
It is also helps to choose colors that suit your subject areas. Cooler colors such as blues and grays tend to suit blogs that are of a technical, political or authoritative nature. Warmer colors seem to suit more casual subjects. For instance a pink and yellow themed site is more suited to a blog about baby showers than it is to a rant about human rights abuses in Africa.
In general the smaller and more classic your font is, the more it implies authority and respect. An example is Times New Roman or Bookman. Arial, Verdana and Helvetica are middle of the road fonts that are offered with most blog templates as a standard font. The shorter, squatter and larger the font, the less professional your blog will look.
Additionally it is best to stick with fonts that most people would have on their computers. Keep in mind that if a computer cannot read a font it will put your site on a default font so it can be read. This can knock your layout out of alignment and give it an unattractive appearance on other people's computers. Fonts that any computer can read include Arial, Verdana, Times Roman, Times New Roman, Georgia, Palatino and Tahoma. Keep your layout as simple as possible to avoid distortion of it on other people's computers as well.
One warning is to stay away from the black hat SEO technique of making your keywords the same color as a web page. The search engine spiders are on to this technique and will penalize your site for it.