This seems to have started with the newest version of iBooks that came out recently. For me, all my old books I converted with Calibre worked fine in the old iBooks, but the newest release puts the blank page in there. I found that if I edit the CSS file for the ebook and remove the property "page-break-before: always" then the book displays the same as before. Calibre adds that property when it converts and it seems either the old iBooks handled it properly or ignored it.
So far hand editing that is the only solution I have found. Means I have several hundred books to edit now.
Update: Actually just got a reply in my thread in the Calibre Conversion forum with a possible solution, check out
https://www.mobileread.com/forums/sho...d.php?t=122510