Let's look at the verse again.
Heb 10:29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?
despite = G1796 = From G1722 and G5195; to insult: - do despite unto.
G5195 = From G5196; to exercise violence, that is, abuse: - use despitefully, reproach, entreat shamefully (spitefully).
account = G2233 =
Middle voice of a (presumed) strengthened form of G71; to lead, that is, command (with official authority); figuratively to deem, that is, consider: - account, (be) chief, count, esteem, governor, judge, have the rule over, suppose, think.
So this means to think, suppose, or esteem the blood of Christ to be unholy, profane, unclean, common, and in doing so exercising violence, abuse against the Holy Spirit of Grace.
So what would common mean in reference to the shed blood of Christ, LESS than even the blood of bulls and goats for a Jew, for gentile LESS than their sacrifices to Pagan gods. UNHOLY = unable to redeem, unable to appease the wrath of God or gods.