この素晴しい世界に祝福を!
As you can see, this phrase is not a complete sentence in a grammatical sense.
The を is here because the verb of this sentence is left unsaid. The omitted verb phrase could probably be something like 送ろう/送りましょう, 捧げよう/捧げましょう or もたらそう/もたらしましょう, etc:
「この素晴らしい世界に祝福を(送ろう/捧げよう/もたらそう etc.)。」
lit. "(Let us/me send/dedicate/bring) blessings to this wonderful world."
... or it might even be (神が)もたらしたまえ or 与えたまえ ("May God bring/give...") etc... I suppose it could be quite open-ended.
The に (lit. "to") is here because of the omitted verb 送る/捧げる/もたらす/与える (lit. send/dedicate/bring/give).
「[人/物]に祝福を。」(lit. "Blessings to [someone/something].") sounds more natural and is more common than 「[人/物]へ祝福を。」. This に indicates the recipient of the blessings, rather than the direction (≂ "toward~~", 「~へ」) of the movement of sending, bringing, or giving the blessings. (In other words, in this phrase the recipient/相手 or the destination/到達点 is more focused over the direction/方向性.) Similar examples:
「これを君にあげましょう。」 (rather than 「これを君へあげましょう。」)
「弟に算数を教える」 (rather than 「弟へ算数を教える」)
「鳩にエサを与えないでください。」 (rather than 「鳩へエサを与えないでください。」)
As for the interchangeability of 「~に」 and 「~へ」 used with 移動を表す動詞 (verbs involving movement):
「東京に行く」「東京へ行く」
「東京に向かう」「東京へ向かう」
「荷物を東京に送る」「荷物を東京へ送る」
Both に and へ are acceptable here, and as a general tendency に indicates the destination of the movement (closer to "to"), and へ, the direction (closer to "toward"). 明鏡国語辞典 also states: "「に」は到達点・相手を、「へ」は方向性を重視した言い方". But the distinction is subtle and they're often used interchangeably in real life. One more difference that I can think of is, the に versions sound a bit more casual and the へ versions more formal.
In Japanese, especially in speech/conversation, the verb (phrase) of a sentence often gets omitted when it can be inferred from the rest of the sentence, eg:
「あ、言い忘れたことが。」
「何か問題でも?」
「助けて!中にまだ子供が!」
「目にゴミが…。」
etc.
And this kind of omission quite often occurs in song/novel/film titles or slogans, too. eg:
『アルジャーノンに花束を』
『まごころを、君に』
『君の名は。』
『ティファニーで朝食を』
『手のひらを太陽に』
『いつも心に太陽を』
『世界の子供たちにワクチンを!』
As for the spelling of 素晴らしい vs 素晴しい, I think 素晴らしい is far more common, especially in contemporary Japanese. And the original title of the manga also uses 素晴らしい:
