how access $content array inside a block php?
If you need to extract the comment of a node and put it in a block you can achieve this with a view.

Create a new view who show 'Comments' and create a block.
Now you can add a contexual filter:

Comment: Nid
The node ID to which the comment is a reply to.

After that select 'Provide default value'
and 'Content ID from URL'

Now you can add a 'Validation criteria'
and select 'Content' and check only the content type you want with this block.
now set 'Action to take if filter value does not validate' to 'Hide view'

Now you can add this block to a region in the block page and you have the node's comment displayed in a block. 

Let me know if this will work for you ^^