Blind Form Submits are still possible. I've just recently posted a topic on that with a generator and instructions. Should be straight forward. Have a look at it, if you have any questions let me know!
You can also follow the steps in this document to create a blind form.After generating the blind form link make sure pass the SYS_Opt-in value as mentioned in the double optin document.
You have to make sure the HTML name of the form ,HTML name of the email address field and the other fields used in the blind form link is same as per the name in the eloqua form.
I see in the document you are referring to the links for blind form submits are outdated (using now.eloqua.com). Use the generator in my previous comment to build the link.
As custom fields, please use the e.g.: "Confirm Opt in / Global Unsubscribe" - fields, please make sure to also copy the HTML names as stated in Renuka's comment.
Thanks, your link generator looks great!
I think it'll be much easier now.
If you have any issues with it -or need help- let me know!