So, I downloaded a texture pack called redux2.zip. The texture pack adds more blocks, but if I go into singleplayer, there are no custom blocks. Just the default ones, some with corrupted textures. How do I enable custom blocks for singleplayer?

Texture packs cannot have custom blocks by themselves - you'd have to define them in a server, or download a world with defined custom blocks and load it into singleplayer or import it onto a server.

53 minutes ago, aleksb385 said:

Texture packs cannot have custom blocks by themselves - you'd have to define them in a server, or download a world with defined custom blocks and load it into singleplayer or import it onto a server.

Well I tried to save a multiplayer map, with custom blocks and load it in singleplayer, but it crashes with this:

Crash time: 02/07/2021 14:23:57
ClassiCube crashed.
Reason: Unhandled exception 0xC00000FD at 0x000000000041FE23 [0x0000000000000001,0x0000000000713FF8,]
Commit SHA: 0421d68
-- registers --
rax=0000000000000029 rbx=0000000000000004 rcx=00000000000000A0
rdx=0000000000000029 rsi=0000000000000049 rdi=000000000669F040
rip=000000000041FE23 rbp=000000000117BA00 rsp=0000000000714000
r8 =00000000000000A4 r9 =000000000000000C r10=000000000003D090
r11=000000000000000C r12=000000000000009F r13=0000000000000030
r14=000000000000003F r15=00000000003955CB
-- backtrace --

Nvm, got it working. I guess the Mp map was too big, so it crashed.

If you turn off Block Physics in Esc -> Options -> Misc Options, you shouldn't experience crashing with large maps anymore (workaround until https://github.com/UnknownShadow200/ClassiCube/issues/812 is eventually fixed)