Why can I not see the profile pictures of the users I blocked?

When you block a user, all the contact and occurrences of that user are blocked/hidden.

The only place you can see the blocked users is in the "Blocked Users" page, under your Account Settings.
You can only see their username, but not their profile picture.

If we displayed the profile picture, that would make it easier for some people stalk their profile picture through that list.
Users usually change their profile picture more often than their username. So, having the picture visible isn't so beneficial.

Note that other social networks usually do not show any profile pictures on the Blocked list.

You can use the "Show Canceled" link, in the "Blocked Users" page, to filter by canceled users.
The temporarily inactive users don't appear there, so any users in the Show Canceled list are generally safe to be unblocked.