Synthetic center.

I can think of two designs that are/were composite centered blades:

For example:
The now discontinued Darker Center Carbon (relatively thick hinoki 5 or 7 ply with carbon center)
Sitco Carboflex Alpha (a 5 play with birch outers and balsa core plys around the carbon - originally designed by the proprietor of Blades By Charlie, now sold here under his brand name: http://www.bladesbycharlie.com/CarboFlex-Alpha)

I'm not sure why these designs "fell out of favor" or were never as popular as dual ply carbon blades. Perhaps a single ply of Carbon lacks some of the torsional stability that a dual ply allows? Twisting of the face would be the wrong kind of flex.

Having had a few hits with both of the above blades, I can say for certain that they allow much of the over the table touch of slower/all wood blades, and have a wicked kick once you engage the carbon.

The best composite that I have played with is the now discontinued Stiga Super Carbon (meaning it has an all wood feel for looping and over the table play, but with distinct blocking and smashing stability of carbon)  - a blade that, like BTY's Innerforce series, buries the two carbon plies deeper into the blade design, under a pair of outer plies.

bluebucket wrote: The ITTF's blade rules are a little strict in this area, back in the 70s full composite blades were being tested but have since been snuffed out

Right.  I believe non-wood plies can only account for 85% of the blades total thickness.  That's from memory.  Check the rules to be sure.  Either way, as bluebucket says, it is simply a matter of the rules as currently written.