Is this a literal translation of an expression in your native language? It's not an expression we'd use in English. The only context I can think of where it *might* make sense is in a video game, where you have to do something to maintain your 'blessed-level'... but that's me really stretching to find a context where it makes sense. If a non-native speaker said it we'd understand that they meant something like "May you be blessed", but it isn't an expression we use in English.

It is very formal and awkward. You could say "bless you."

There's nothing wrong with this grammatically, but it doesn't make any sense. One reason it is wrong is that we can only say 'Keep ...' if the person has some kind of control over the situation. For example, we say 'Keep calm' or 'Keep quiet' when we are telling someone not to get upset or make a noise. The state of 'being blessed' is not something that you can control in this way.