Is the repentance of one who blasphemed Allah in his youth, ignorant of the gravity of such an act, accepted if he later regretted it and repented sincerely after realizing the enormity of his sin?

What you did is a grave sin for which its perpetrator is excommunicated and asked to repent. If he repents, good; otherwise, he is to be killed. To learn how to repent, please refer to Fatwa No. 8927. If you repent, feel remorse, and resolve not to return to it, you will be truthful, and Allah will overlook your sin, forgive you, and rejoice in your repentance. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" (Quran 39:53). And He Almighty says: "And turn to Allah in repentance, all of you, O believers, that you might succeed." (Quran 24:31). This applies if the act was committed after puberty. However, if it was before puberty, there is no sin upon you, for the Prophet, peace and blessings of Allah be upon him, said: "The pen has been lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane person until he regains his sanity."