Openssl magic values? Secp256k1 and ECDSA similarities? What makes secp256k1 special?
You are using the eliptic curve function of openssl and providing a serialized bytestring as an input. DER (Distinguished Encoding Rules) is a restricted variant of BER for producing unequivocal transfer syntax for data structures described by ASN.1. 

The ASN1 structure for a privkey looks like this:

# ASN.1 STRUCTURE FOR PRIVATE KEY:
#   30  &lt;-- declares the start of an ASN.1 sequence
#   74  &lt;-- length of following sequence 
#   02  &lt;-- declares the start of an integer
#   01  &lt;-- length of integer in bytes (1 byte)
#   01  &lt;-- value of integer (1)
#   04  &lt;-- declares the start of an "octet string"
#   20  &lt;-- length of string to follow (32 bytes)
#           3cd0560f5b27591916c643 ... a738d2e912990dcc573715d2c 
#           \--------------------------------------------------/
#            this is the private key 
#   a0   &lt;-- declares the start of context-specific tag 0
#   07   &lt;-- length of context-specific tag 
#   06   &lt;-- declares the start of an object ID
#   05   &lt;-- length of object ID to follow 
#   2b 81 04 00 0a &lt;-- the object ID of the curve secp256k1


See also:
Signing transaction with SSL - private key to PEM