From Wikipedia:

In propositional logic, transposition is a valid rule of replacement that permits one to switch the antecedent with the consequent of a conditional statement in a logical proof if they are also both negated. It is the inference from the truth of "A implies B" the truth of "Not-B implies not-A", and conversely.It is very closely related to the rule of inference modus tollens. It is the rule that:

P -> Q <=> (~P -> ~Q)

This means that you can replace one statement for the other. What the ravens paradox shows us is there is a flaw with induction, which is what we use everyday to make sense of the world.

Assuming that every time that it rains we see that it must be cloudy, we can see that:

Raining -> cloudy
Not raining -> not cloudy

How could it rain unless it is cloudy?

When you say it can be cloudy but not raining, notice the order is different. It could perhaps be snowing when it is cloudy or no precipitation happens. But that doesn't take away from our first statement that:

Raining -> cloudy

We obviously must be careful with this, because using this logic we showed that all ravens are blue, which is obviously false.