The Blackfin processor was designed by and is manufactured by Analog Devices (Board and Chip Vendors#A). Several boards that include a Blackfin processor are available (Hardware Hacking#Blackfin).
All Blackfin processors include Debug/JTAG Interface for in-system debugging.
Blackfin processors are fast enough to support real-time H.264 video encoding.
The Blackfin processor documentation describes a Memory Management Unit (MMU), but this is not a MMU in the sense that most people expect: it has no Virtual Memory (VM) support. It does however provide all other aspects associated with your typical MMU on a variable page sized basis such as Memory Protection (read/write/execute) and caching. The documentation refers to these mappings as the Cacheability Protection Lookaside Buffer (CPLD) tables and are similar to TLBs in most other processor architectures.
As such, all current Blackfin parts can only run Linux with MMU support turned off (what people have historically thought of as "uClinux").
The processors also fully enforces privileged execution -- the classical user / supervisor permission split. The Linux kernel runs in supervisor mode where it has full access to the hardware while the Linux userspace runs in user mode and prevents tampering with hardware resources.