Nothing really. Common sense says that the tempos are usually between 100 and 200. There are no official rules to determining whether a song is in 120 BPM or 240 BPM, but what I use is the overall feel of the song, especially the speed of the drums. Another thing to keep in mind is that 32nd notes are somewhat rare, so if you have lots of sections with 32nd notes the BPM should probably be doubled. None of these are definitive. I have a song that I wrote the intro melody to in 110 BPM as it was really slow yet made the rest of the song pretty fast and never bothered rewriting the intro in double the tempo so I left the tempo at 110. Were I to start writing tabs for said song now that I know the whole of it I would probably do it in 220 BPM. As for blast beats and changing tempo? Don't change the tempo. Ever. Until you know what you're doing. 95% of songs have the same tempo throughout. I don't think Bodom has done a single song since HB that has a tempo change in the middle of a song (Towards Dead End has one before the solo). So when doing blast beats or fast stuff on guitar just switch to 16th or 32nd notes.