It's a 4th tone. However, when it comes before a 4th tone, it changes to a 2nd tone. The Pimsleur program explains that. So do all the books.

bú and bù are both right . Most cases, 不 is used to be part of a fixed phrase . like 不是，不好，不行,etc The rule is, when 不is followed by a 4th tone, it should be procounced "bú" , like you said :wǒ bú shì when 不is followed by a 1st, 2nd ,3rd tone, it should be pronouced "bù" ,like : 不依不饶 bù yī bù ráo ； 不好 bù hǎo Otherwise, 不 bù = no

Actually, you can find three kind of tones when chinese people say this word 不(bu). 1, 不bù: the main tone Generally, the basic tone of this word 不 is the forth tone. For example:不 (bù), 不好(bùhǎo), 不想(bùxiǎng), 不明白(bù míngbai) 2,不bú: tone sandhi When a 不(bu) comes before a forth tone word, this 不 has to change its tone to the second one. For example:不对(búduì), 不用(búyòng), 不要(búyào) 3,不bu :the neutral tone when this word 不(bu) is sandwiched between two words, we usally read it in a light voice. For example:对不起(duìbuqǐ), 好不好(hǎobuhǎo),

中国文字有很多多音字，还有一部分虽然不是多音字，但其声调随语言环境不同而变化，如“不会”，“不要”中的“不”读二声“bú”，而“不好”，“不能”读“bù”,，“好不好”中的“不”读“”这就是“不”的“变调”，这种变调是由规则的，也就是在四声前读作二声，其他读作四声，还有部分情况读轻声，如“好不好”。（你可以查看“一”“七”“八”“不”的变调） 有机会我也可以给你讲讲，但我的英语很差。 Chinese characters have many polyphone, partly though not polyphone, but its tone with the language environment vary, such as "no", "don't" of the "no" read two sound "bu" and "bad", "not" read "bu",, "good" of "no" read "" this is" no "" this tone sandhi tone sandhi," by the rules of the tones, namely before read two sound, other read tones, and parts of situations, such as "read softly good". (you can check "one" "seven" "eight" and "no" tone sandhi) I have the opportunity, I can tell you about, but my English is very poor.

I agree with Eliot.