So I was wondering, how do japanese people greet each other when they are, for example, writing on a blog or recording a video? My doubt comes from the fact that their greetings will be directed towards someone who is unknown, and that the greeting will be seen on a unpredictable time. Someone who accesses the blog (or watches the video or whatever) could to that at anytime of the day. Does that even matter? Or one should take into consideration the time when they're are writing/recording? Or neither?
As a generic you can't go wrong with こんにちは. For example on Facebook or twitter I see posts by companies often that start with こんにちは. It depends on the context, of course, so if there's a situation where time of day is just understood then you can use that greeting. Or you can give a twist to it and start with ハロー！
A good reference for you would be a Japanese podcast.
Every Japanese greeting with new people, whether business, audience or casual encounter, involves,
Salutation Ex: こんにちわ、おはようございますetc.
Self introduction, where you belong, your connection to the group or what you do. Ex: ＡＢＣサイト中国語担当の田中と申します。宜しくお願いします。
Can you be more specific? Keep in mind, for every situation or season, there's a set routine Japanese people do things. Context is king in speaking Japanese.
Everything depends on the audience.
In Japanese there are two distinct types of speech:
-Formal speech, to be used with strangers, people older than you and family members you do not know well.
-Informal speech, to be used with close friends, family members and those who are the same age or younger than you.
In a blog or video you might want to break the ice with the reader/viewer and might adopt an informal tone so おはようございます will be おはよう and so...