Ironically, given the title being addressed, I personally wrote for Helium before I ever wrote a blog, although I would argue that I apply the same grammatical rules and lack of abbreviation there as I would here. Therefore I transitioned from writing for Helium to writing for blogs.
Fundamentally, though, in my first paragraph I have come close to committing the first basic error in writing for Helium, I have emphasised "I" over the third party. The skill of writing for Helium is to produce considered arguments in the cogent writing style that citizen journalism requires. When you write for Helium you step back from the anecdote unless it is pertinent and does not wander from the point of your article. You can feel impassioned about a subject for sure, but you must back this passion up with facts and reason in a way blogs do not require.
Helium gives a voice to those who may be experts in their field but not necessarily jourmalists, and writer who can provide short researched pieces of writing on a certain subject to inform and entertain others. Blogs, although often entertaining and written by people who are often passionate about their own subjects have no obligation to be reasoned to argue their point, and have carte blanche to "rant" if they feel the need.
If you write in a blog or in a chat room making your point is all that matters, you can do this using abbreviations, smileys and grammar although it may cloud your meaning, lapses in grammar or spelling are acceptable. At Helium, the articulation standards are far higher, however, both share the need to create small, succint blocks of information, "white space" for web readers.
Referencing is also important. If you are writing a product review then mentioning websites that feature these products is useful and helpful, however when quoting facts or beliefs you are best placed referencing the ideas that you are using in order to avoid any accusation of plagariasm.
Structure is another main difference, at Helium you are fairly tightly tied to the content title you have chosen, and as such it is best to have a precis of your argument at the beginning, followed by a structured content and a conclusion to follow to tie up any loose ends.
Writing for Helium is a discipline like any other, once you get the hang of it you will wonder how you ever wrote in any other manner. I often find that the pieces that I write that do best are the ones that I have approached more dispassionately and that the things I really care about, I have allowed my judgement to cloud me, and I haven't given it the distance it needed.
The transition from one to the other need not be painful, it just needs an understanding of what each media is designed to do and the need it fulfils.