Young ones often are blessed by the parents, grand parents etc etc. the blessings are rendered useless if everything forms part of Karma?
Nothing goes useless, every Karma gives new Karma. It is cycle that cannot be broken, But the Blessings can change result time of the Karma. It is depend on how powerful blessings against the Karma.
I am sorry I didn't find any written evidence. But you thing a deep (manthan) in it, you will get same as I told.
Can you write a cheque for more than the balance in your account? Sure - but you will eventually get into trouble for writing bad cheques!
I personally feel it is the same with blessings. When giving a blessing one should be careful to limit one's expression to that which is within ones power to give. While it is always possible to wish someone well, it's just that your good wishes may have no impact on that person's life if your wishes are not backed by your own karma!
One gets a blessing or boon from another, when s/he is eligible according to their karma. Otherwise, one can not get a blessing/boon. Every event follows law of karma.
Good karma gets good blessings. Evil deed gets curses.
Getting a blessing/curse is a part of karma.
It is not possible to get the fruits of a blessing, without a good deed or without eligibility. This rule applies to not only humans or daemons, but also to angels and gods.
The whole idea behind Bhakti is that God is more powerful than Karma. Similarly our elders and Acharyas have powerful blessings. If Karma were all powerful in Vedanta then Hinduism would be no different from Jainism.
In Srimad Bhagavatam 6 2 17, we find,
Although one may neutralize the reactions of sinful life through austerity, charity, vows and other such methods, these pious activities cannot uproot the material desires in one’s heart. However, if one serves the lotus feet of the Personality of Godhead, he is immediately freed from all such contaminations.