That's because you're dealing with an INSERTion of the block, not the block definition.
To get the block definition, you need to use (setq temp (entget (tblobjname "block" "PipeDownMedium"))) ...then (entnext temp) will get the next element. ___
> Command: (entget (setq temp (car (entsel))))
>
> Select object: ((-1 . ) (0 . "INSERT") (330 .
name: 40611cf8>) (5 . "51F") (100 . "AcDbEntity") (67 . 0) (410 . "Model") (8 .
"P-SAN-PIPE-N") (100 . "AcDbBlockReference") (2 . "PipeDownMedium") (10 85.3438
> 185.219 0.0) (41 . 1.0) (42 . 1.0) (43 . 1.0) (50 . 0.0) (70 . 0) (71 . 0) (44
> . 0.0) (45 . 0.0) (210 0.0 0.0 1.0))
>
> Command: (entnext temp)
> nil