Solution:
Place this code in your DOCUMENT_ROOT/.htaccess
file of old domain:
RewriteEngine On
# URIs without any p=... query string
RewriteCond %{HTTP_HOST} olddomain\.ca$ [NC]
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^blog/(.*)$ http://www.newdomain.com/$1 [NC,NE,R=301,L]
# URIs with p=... query string
RewriteCond %{HTTP_HOST} olddomain\.ca$ [NC]
RewriteCond %{QUERY_STRING} (?:^|&)p=([^&]+) [NC]
RewriteRule ^blog/?$ http://www.newdomain.com/index.php/p%1? [NC,NE,R=301,L]