so any motherboard with only a blue and a black pcie slot is not sli compatible?
Probably not. The black slot on the bottom of the board is tied to the PCH (formerly called the Southbridge) which is the IO part of the Intel chipset. It only runs at PCIe 4x despite having a full slot size. The blue slots are tied to the CPU directly. Communication between a black and blue slot would require that the data make an additional bus hop as it would have to go through the DMI bus that connects the CPU and PCH.
The black slot also shares PCIe bandwidth with a couple of add-in controllers such as the USB 3.0 controller and additional SATA-III ports.
There is no technical reason why it can't be done, because this is exactly how NVidia used to do it when they produced chipsets themselves. When NVidia first introduced 3 way SLI with the NForce 700 series chipset two of the GPUs would be connected to the Northbridge and one would be connected to the southbridge.
I suspect that NVidia may not want SLI implemented in this fashion anymore due to the bandwidth issues that can arise, especially when a GPU ends up sharing bandwidth in a low bandwidth slot.