Since memory technology is developing very fast it is very hard to find and update product.
I have some experience with Blackfin BF33x and now plan to migrate into newer BF51x series.
There are two EE-Notes on selection of SDRAM;
EE-210 SDRAM Selection Guidelines and Configuration for ADI Processors (year 2004)
EE-326 Blackfin Processor and SDRAM Technology (year 2008)
I also looked the Engineering Zone;
The recommended mobile SDRAM in EE-326 and Engineering Zone forums is;
and it is OBSOLETE!
The parts in EE-210 and Evaluation Board for BF518 are also OBSOLETE!
There are many SDRAM technologies;
SDRAM (most are obsolete)
Mobile LPDRAM (most are obsolete)
Especially PSRAM/CellularRAM seems to be promising and according to Micron's application note TN-45-27 "Using Micron Async PSRAM with ADI Blackfin Introduction" it is possible with most of the Blackfin DSPs.
Which non-obsolete SDRAM technologies above are compatible with your Blackfin BF51x series DSP?