If you want to do it manually, without lapo.it, just search your DER encoded private key from the beginning for "04 20" and grab the following 32 bytes (64 hex characters). Example:
30820113020101
0420
c4bbcb1fbec99d65bf59d85c8cb62ee2db963f0fe106f483d9afa73bd4e39a8a <<<< Give this to blockchain.info
a081a53081a2020101302c06072a8648ce3d0101022100fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f300604010004010704410479be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8022100fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141020101a1440342000478d430274f8c5ec1321338151e9f27f4c676a008bdf8638d07c0b6be9ab35c71a1518063243acd4dfe96b66e3f2ec8013c8e072cd09b3834a19f81f659cc3455
EDIT: I'm assuming you're getting the DER encoded key from somewhere other than brainwallet, as obviously if you already have it on brainwallet you can simply copy the Secret Exponent directly into blockchain.info.