What's a good translation for 说法？ 在我们这边是有这种说法——混血儿会比较聪明。


I know several meanings for it.

官方说法, Official statement;

讨说法, seek explanation or justice.

今日说法, talk about law.

However, here 说法 means a kind of public opinion. Can it be translated to saying which convey the 说 part meaning?
Yup, in this context, the translation of saying or common saying would work.
Besides the several meanings you pointed out, 说法 also has another meaning that is commonly seen: wording or the way that something is said. An example is:

你刚刚说得有点太复杂，换一种说法意思可能清楚些。
The way you were explaining was a little too complicated, perhaps changing the wording might make it clearer.