Get config variable value in hook_block_list_alter
Assuming you've set the variable using the variable_set() function, you can get the value using the variable_get() function.
From the docs:

Returns a persistent variable.
Case-sensitivity of the variable_* functions depends on the database collation used. To avoid problems, always use lower case for persistent variable names.

In your case the foreach loop would become :
foreach ($blocks as $key =&gt; $block) {
  $enable_conectivity = variable_get('enable_conectivity', TRUE); // Default value of `TRUE` is returned if the variable is not set.
  if($enable_conectivity) {
    $blocks[$key]-&gt;visibility = BLOCK_CUSTOM_DISABLED;
  }
}