I must admit that, even moreso than Bluelight, the Vaults of Erowid has most profoundly influenced my understanding - and opinion - of drugs in general; this influence, for me personally, has always lead to a more positive (happy/healthy) consideration, and use, of recreational drugs. My bias, to be fair, is that I discovered Erowid when I was 14 years old – the same year I became interested in drugs, an interest for which Erowid was partially responsible.
I found Bluelight, on the other hand, when I was 18 and already seriously abusing a variety of drugs; at the time I was particularly addicted to uppers (Rx stimulants and cocaine). My opinion on the matter (drugs, addiction, and related things) was already fully-formed - or so I thought, until the following year rolled around and I suddenly realized that my perspective had completely changed somewhere along the line).
It’s also true that Erowid offers greater depth than Bluelight in terms of thoroughly addressing a subject (probably due in large part to the fact that Erowid is an endless database, while Bluelight is just a forum) from various angles, and drawing from various professional fields, cultures, time-periods, etc. Erowid documents the way in whitch drugs work, and the way in which they effect us (both in mind and body). And, even more importantly perhaps, Erowid discusses the way in which people, and societies, consider, and interact with, drugs –as it relates to the individual, to the society, and to the species. Erowid's mission is stated simply: “documenting the complex relationship between humans & psychoactives.”
Bluelight, however, is clearly the more "simple" of the two. It is less comprehensive and covers less fields of study; the study of drugs itself, as addressed and presented by Erowid, on the other hand, draws from and incorporates such disparate fields as anthropology, sociology, biology, chemistry, philosophy and religion, among others. Yet, Bluelight is truly the greatest resource/database of its kind, championing the goal of documenting the “drug culture” and related phenomena through the lens/content-filter of "harm reduction" – its circles and members, its places and means, its slang and mythology, etc. Ultimately, as a forum, Bluelight is the sum total of the ongoing interactions between members of the drug world.
To summarizes the above in a nutshell (tl;dr) : Bluelight documents how drug users interact with one another, whereas Erowid documents how drug users interact with drugs (and vice-versa).