Data at rest on backup storage side or primary storage?

Data at rest will be on primary storage.

Whenever you read data it will need to be decrypted and that may have impact on performance, but modern solution usually relay on fast HW based cards sitting between disk and software layer like application so impact is much less to be of any concern.  If using software based encryption it might have some impact, but it shouldn't be that high nowadays.  I guess, before you go live you will have proof of concept and test of this kind involving backup and restore should be performed and compared.

Does that mean I have to decrypted the data before I do any backup?

Is it possible to leverage snapshot at the block level so that I do not have to decrypt my data before backup?

You didn't say how it will be encrypted in first place, but I assume same process which encrypts data during write will run decrypt during read and this should be automatic.

The encrypt and decrypt by the same process used by the backup should be the norm but that backup of the SQL DB will be in plain text.

I looking for solutions to backup the SQL DB with encryption as it is.

Any possible solution will be helpful.

Thanks.

As it is - where?  On disk from OS or application point of view, on disk array from its point of view?  If you are transferring data from A to B, which is what you do with backup, then either this is done online where you must decrypt it or offline where you do not have necessary do that, but such backup would be offline (eg. snapshot of encrypted data on disk from OS/application point of view) and could only be restored by original client most likely (in presence with original crypt routine).