a shot in the dark 瞎猜;盲干

【例句】
The figure he came up with was really just a shot in the dark.
他得出的数字实际上只是瞎猜而已。

We had nothing to go by. It was a shot in the dark, but it turned out right.
我们是没有根据的，只不过瞎猜，可是结果却猜对了。