In order to be actionable, a defamatory statement must be "of and concerning" the plaintiff. This means that a defamation plaintiff must show that a reasonable person would understand that the statement was referring to him or her. Of course, if a blog post or online article identifies the plaintiff by name, this requirement will be easily met. The plaintiff need not be specifically named, however, if there are enough identifying facts that any (but not necessarily every) person reading or hearing it would reasonably understand it to refer to the plaintiff. For example, a statement that "a local policeman who recently had an auto accident had been seen drinking alcohol while on duty" would likely be actionable because the policeman could be identified based on his recent accident.
Accordingly, defamatory statements about a group or class of people generally are not actionable by individual members of that group or class. There are two exceptions to this general rule that exist when:
the group or class is so small that the statements are reasonably understood to refer to the individual in question; or
the circumstances make it reasonable to conclude that the statement refers particularly to the individual in question.
See Restatement (2d) of Torts, § 564A (1977).
As to the first exception -- statements about a small group -- courts have often held that an individual group member can bring a claim for defamation for statements directed at a group of 25 or fewer people. The 25-person line is not a hard-and-fast rule, but rather the way courts commonly distinguish between a group small enough for statements about the whole group to be imputed to individual members and one that is too large to support such an imputation.