Breeding two porcelain together will give you all porcelain nothing else.
Breeding a self blue to a porcelain will give you zero porcelain. Imo unless you really know your genetics all youll get is a mess.
To get porcelain you need to get the lavender gene from each parent but you also need to get the MF pattern from each parent. You can't get that from the self blue.
Self blue is just extended black underneath the lavender.
You can do that cross and with the right breeding get back to porcelain in a couple generations but you will need to understand all genetics of both colors/patterns.
I believe you are thinking BBS blue, not self-blue. Most folks who use the term self-blue are referring to the lavender gene, not BBS blue. Some breeds call it lavender, some call it self-blue.