How to drop what follows "can (be)" to avoid repetition?

Question

It is grammatical (I think) to say: "My sister can sing and I also can". But are there correct sentences among the following ones:

1. Some berries cannot be eaten raw, but blackberries can
2. Some berries cannot be eaten raw, but blackberries can be
3. Blackberries can be eaten raw, and blueberries also can
4. Blackberries can be eaten raw, and blueberries also can be

Answer 1

All of your sentences are a varying degree of correct, at least in common usage. The "proper" construction would be:

Some berries cannot be eaten raw, but blackberries can be eaten raw.

Depending on regional variation, most native speakers wouldn't repeat "be eaten raw" in its entirety, ending on either "can" (which in my experience is more common) or "be," as you've listed above.

Answer 2

In all of your example using the word 'can', put 'can' at the end of the sentence:

"My sister can sing and I can also."
"Blackberries can be eaten raw, and blueberries can also"
"Blackberries can be eaten raw, and blueberries can be also"

Although these would be better expressed as :

"My sister can sing and so can I."
"Blackberries can be eaten raw, and so can blueberries."

Apologies but I can't give any grammatical explanation for these constructions; they are just more natural to a native English speaker.