-Why should we continually ask forgiveness?
Because we, as people, are continually sinning. We are not perfect, and God understands us, but He has also set out the Law. Jesus said that if a person even thinks about breaking one of the Laws, then he has already done it in his mind, and is therefore guilty of sin.
-How can unforgiveness on our part block God's blessing?
Unforgiveness blocks God's blessings from us because we are not following His commands. If we, as parents, take away a child's toy, it is to teach that child a lesson. Withholding a part of His blessing is God's way of disciplining us, to teach us a lesson.
-How can unforgiveness block God's forgiveness?
If we are not willing to forgive others, what makes anyone think that they would deserve forgiveness? Granted, nobody actually does deserve forgiveness, but God still gives it as a gift. And what God does give, He also has the right to take away.