Both aliquid bonum (aliquid + neuter nom/acc. adjective), and aliquid boni (aliquid + genitive neuter 2d declension adj. or gen. noun) are idiomatic.
From Lewis & Short aliquis:
B Not unfrequently with adj.: Novo modo novum aliquid inventum adferre addecet, Plaut. Ps. 1, 5, 156: novum aliquid advertere, Tac. A. 15, 30: judicabant esse profecto aliquid naturā pulchrum atque praeclarum, Cic. Sen. 13, 43: mihi ne diuturnum quidem quidquam videtur, in quo est aliquid extremum, in which there is any end, id. ib. 19, 69; cf. id. ib. 2, 5: dignum aliquid elaborare, Tac. Or. 9: aliquid improvisum, inopinatum, Liv. 27, 43: aliquid exquisitum, Tac. A. 12, 66: aliquid illustre et dignum memoriā, id. Or. 20: sanctum aliquid et providum, id. G. 8: insigne aliquid faceret eis, Ter. Eun. 5, 5, 31: aliquid magnum, Verg. A. 9, 186, and 10, 547: quos magnum aliquid deceret, Juv 8, 263:
. . .
D Aliquid (nom. or acc.), with gen. of a subst. or of a neutr, adj. of second decl. instead of the adj. aliqui, aliqua, aliquod, agreeing with such word: aliquid pugnae, Plaut. Capt. 3, 4, 54: vestimenti aridi, id. Rud. 2, 6, 16: consilii, id. Ep. 2, 2, 71: monstri, Ter. And. 1, 5, 15: scitamentorum, Plaut. Men. 1, 3, 26: armorum, Tac. G. 18: boni, Plaut. Aul. 4, 6, 5; Ter. And. 2, 3, 24; Vulg. Joan. 1, 46: aequi, Ter. Ad. 2, 1, 33: mali, Plaut. Ep. 1, 1, 60; Ter. Eun. 5, 5, 29: novi, Q. Cic. Pet. Cons. 1, 1; Vulg. Act. 17, 21: potionis, Plaut. Men. 5, 2, 22: virium, Cic. Fam. 11, 18: falsi, id. Caecin. 1, 3: vacui, Quint. 10, 6, 1: mdefensi, Liv. 26, 5 al.—Very rarely in abl.: aliquo loci morari, Dig. 18, 7, 1.—
http://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.0:1876.lewisandshort
Aliquis/aliquid shouldn’t be confused with aliqui/aliqua/aliquod.
Aliqui/aliqua/aliquod is an adjective and needs a noun to modify. In this sentence, “Non est tibi quidquam boni, sed aliquod malum,” aliquod malum would correspond to English “some evil”, not “something bad/evil”.
Aliquis/aliquid is a pronoun and, like English “someone/something”, can be modified by an adjective or, in the case of aliquid, take a genitive noun or neuter 2d decl. adjective, as the examples show. Aliquid boni/mali is like French quelque chose de bon/mal, Italian qualcosa di buono/male.
NB: aliquis can also be modified by an adjective (I cut off the examples of this in L&S in order to focus on aliquid). Unlike aliquid, aliquis can’t take a genitive adjective complement. Aliquid can apparently only take a genitive second declension neuter adjective, according to L&S–probably because the genitive neuter form of a third declension adjective would be identical to the nominative masc./fem. form, which would be confusing, e.g., *aliquid terribilis. Only aliquid terribile would be possible: “something frightening.”
Check Lewis and Short for idiomatic usages like this.
Hope this helps.
Edited 7/19.