How do I write a cover letter for a journal that has a double blind review process when I need to reference my previous conference paper? My research area is in Computer Science. 

I am going to submit a paper to a journal, that has a double blind review process. The paper builds upon our previously published work in a conference paper. It has over 40-50% new content, experiments and results, and less than 30% textual similarities. This is allowed by the journal (according to their website)

I need to reference the original conference paper in the journal paper and say the current submission has X,Y,Z content, which are new and not included in the conf. paper. I would also like to write a cover letter stating that this submission contains new material, over the original conf. publication, outlined in bullets. I would like to cite the conf. paper in the cover letter too.

The journal says, all indication of author names should be blanked out. Hence, their guidelines are a bit conflicting. 

So how can I write the cover letter without giving the author names away ? How can I point to the original conference paper without giving away the author identity ? 

Many thanks for your help.

EDIT:
I checked the journal website, it says :

"...Of course authors  have  rights  to publish in journals enhanced  versions of a their conference presentations. Such papers however should be submitter as regular papers. Obviously, the conference paper must be cited and it should be explained how the conference paper has been enhanced. The lack of this citation is considered a serious violation of ethics and as a 
consequence the manuscript must be rejected..."

http://tii.ieee-ies.org/o/SubPostConf.pdf
The editor is not subject to the double-blindness and is usually the only one to see the cover letter. They have to know who the authors are so that they can reconnect with you once reviews are in, not try to use any of the authors as reviewers, etc. I'd submit a normal cover letter and make sure that the paper itself is properly blinded. You must cite the prior conference article you are extending, but you have to do it in the third person (e.g., "...in Rosh, et al. they show that..., in this article, we extend their work to show that ...", etc.). 

If you are still confused about this issue, contact an editor at the journal and ask for clarification.