Allowing cookies on some sites

Seems you can easily turn "Block all Cookies" off ... just turn it off the same way you turn it on.

Clear the history, cache, and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

Eg: says:

To block cookies:

1. Go to Settings > Safari > Advanced.
2. Turn on Block All Cookies.

If you block cookies, some web pages might not work. Here are some examples:

• You will likely not be able to sign in to a site even when using your correct username and password.
• You might see a message that cookies are required or that your browser's cookies are off.
• Some features on a site might not work.