For your example,
我们一块去 or 我们一起去
they are of exactly the same meaning. One slightly difference could be that the latter one is more formal (used in written Chinese) while the former is rather oral. And in spoken Chinese, we usually add an "儿话音" to the end of “一块”:
一块儿 (Note: the Pinyin is Yi Kuair, not Yi Kuai Er)
So when referring to
together, you can either use 一起 and 一块儿, but I prefer to use 一块儿 in the daily life.
In other occasions, 一块 can be of other meanings. For example,
一块钱 means "RMB One Yuan"
一块蛋糕 means "a piece of cake"
一起 also has some other meanings:
一起车祸 means "a case of car accident"
Here 块 and 起 are measure words.