This is a question I've had since I first played this game, maybe twenty years ago. I would say that it is random, given the nature of the game. However in my experience; the O-blocks never come alone (after you get one, the chance of getting another seems more likely). I also feel that you get the I tetromino just after you made a mistake.
I've been googling this for quite some time and haven't found a conclusive answer. Can somebody please help me? I've only played the game on the original Gameboy.
Best Answer
This is kind of a vague question and the primary answer is going to be equally vague: it depends on which Tetris you are playing.
More elaborately, every different version of Tetris will have its own block choice algorithm. The ones I've most commonly heard about are a "true" random generator that doesn't make any kind of judgement, a "fair" generator that generates random sequences of all seven blocks (so you /will/ get every type of block equally often, but the order in which you get the blocks within a 'batch' of six will vary)...
... And then there is Bastet, or "Bastard Tetris", which distinguishes itself by having an algorithm that chooses the worst possible block with malice aforethought.
For further reading I recommend the wiki pages here (different tetris random generators) and here (the 'official' guideline to what makes a Tetris game).