Well the Cisco XENPAK 10GB-LR is spec'd for spans up to 10 km.
Per its data sheet, the transmit power ranges from +0.5 dBm max down to -8.2 dBm min. Receive levels must be no greater than +0.5 dBm and no less than -14.4 dBm.
The HP BLc 10GB (assuming it is the LR type transceiver) does not have any such detailed specification that I could find.
I'd estimate worst case planing would would be the difference of the minimum Cisco Tx and the least allowable Rx power = 6 dB loss end to end. When in doubt, measure it!
