Is there an efficient way to hide the encrypted plaintext length with a block cipher?
Counter mode (CTR mode) seems like it will do what you want.  The length of the output is equal to the length of the input, plus the length of the IV.  If this is not what you want, you'll need to explain your question/requirements more clearly.

In practice, I recommend you use an authenticated-encryption mode, such as EAX or GCM etc.