Definite article "the"
 In the first sentence, the statement is quite generic and it does not provide any kind of specific information that could narrow down the universe of blogs being discussed. Therefore, you cannot use the definite article here.Blogs are being used for many different purposes these days.
The first personal computers were very large and heavy.
 Use the with nouns modified by ranking or ordering expressions such as "the first", "the second", "the third", "the next", "the last", "the previous", "the following", "the penultimate", etc.
For more information regarding the proper usage of articles in English, visit: http://www.davidappleyard.com/english/articles.htm