ML, DL & BL indicate the families for HPs lineup of Industry Standard Servers: ProLiant.
BL = Blades= HP BladeSystem.
Up to 16 server &/or storge blades + Networking, sharing power and cooling in 10u enclosure
DL = Density LIne = Rack mount ProLiant server.
Here, HP puts as many features as possible in as small an enclosure as possible. 1 U (rack unit, 1.75") is the smallest. It is a complete server and does not share components (like power and cooling) with other servers (like blades)
ML = Maximized Line. These are typically ProLiant servers in
a tower enclosures but there are several in rack enclosures. Here HP gives you more slots for option cards and more drive bays and typically, more memory slots. They are also complete servers that do not share components like blades.
Depending on the exact model, they have Intel or AMD processors, single CPU, multi-cpu, hot plug components, etc.