A good thing about Black and Decker electric lawnmowers, at least mine, model 875, is that the plastic cover has no holes where water can get on the motor. AFAICT, if you leave it out during the rain, the wheels get wet, and the switch on the handle gets wet, but the plastic cover covers everything else.
I have a spare switch so if I remember, I'll check how waterproof that is, but I'll bet it's pretty good.
Not that i know what this all means first hand. I've kept my mower covered by a plastic cover and under the deck.
And I don't remember their advertising making any claims about this. If true, I think it would be a big selling point. Esp. since most who buy the mowers have small yards and not a lot of space to store them.