[fecak]

It's called a recommendation. Generally speaking, when people recommend something,it's a positive. References might include a negative, so long as you ask a question that allows for a negative answer. When doing references, I usually ask for a potential 'area for improvement' so I am fairly certain to get at least one potential concern.

Being that the person requesting controls whether the recommendation is published, we shouldn't expect to see much negative. A well-written recommendation (even on LinkedIn), written by someone with some level of industry credibility, can actually be a fairly powerful tool to get noticed.