My understanding is that the name dd was essentially a joke. A DD statement in JCL defined a logical name (seen by the program) to refer to a file (`dataset'). It had many strange operands, and often its behavior was completely unintuitive. For example, here is a job that deletes the file MYFILE.

  //MYJOB  JOB
  //FOOBAR DD DSNAME=MYFILE,DISP=(,DELETE)
  //STEP1  EXEC PGM=IEFBR14

IEFBR14 is a program that does nothing at all!

Although the Unix dd command wasn't patterned on the JCL command, I suspect that the multiplicity of possible options led its designers to choose the key=value option syntax that looked vaguely OS/360ish.

By the way, the - flag for options first appeared in MIT's CTSS, which was the direct ancestor (at least at the user level) of Multics.

I'd ... thought that more of the Unix arguments (block, unblock, conv, etc.) were supported on JCL, though my IBM link doesn't support that.

I'll see if I can find a more canonical / complete reference.