Blade technologies really help with the density issue in datacenter, which naturally make them a choice for virtualization. While there has been a focus on using blades in virtualization, there is nothing technically inherent about them that would require their use in virtualization. Still, the form factor, chassis backpane integration, and other benefits do fit well into the virtualization model.
If you are asking in terms of vendor support. VMWare ESX is totally transparent about blades. For storage, there are a bit more options and configurations. NPIV support is a huge step in the right direction.