I wanted to clarify whether it is really true that I may not recite the blessing of leishev basukkah when I see…

I wanted to clarify whether it is really true that I may not recite the blessing of leishev basukkah when I see clouds gathering and darkening, which are a sign that rain will fall soon 
 Could this be? After all, we hold in the Gemara: "עד שתסרח המקפה" — “until the mikpeh dish spoils”?

Shalom u’vrachah 

Even when there are clouds, one may recite the blessing as usual. 

The exception is if one is concerned that heavy rain will begin immediately, and that before he eats a kebeitzah of bread or mezonot foods he will already have to leave the sukkah. In such a case, he is placing himself in a situation of a possible blessing in vain. 

Shulchan Aruch, Orach Chaim, siman 639