"Keep blessed" is not something you are likely to hear in English. Forget this phrase. "Stay blessed" on the other hand, is a phrase you might here. It's usually spoken with religious belief in mind, but can also just be a generic phrase. If you say this to someone, you are expressing that you would like for them to continue to have good things in life. You would like for them to stay safe and feel comfortable. If you say it with the religious beliefs in mind, you are expressing that you believe that your deity of choice is watching over the person (more or less with the same sentiments previously mentioned).