Do I need the Black Magic for Logic?

It "enhances" graphics and can take some processing, but Logic cannot use it for that at all. So maybe if you do heavy pro movie editing or are a hardcore gamer it may be of some questionable use -ish. Also, the "advantages" appear to be marginal, at quite some cost:
"Yes, you can boost your Mac with Blackmagic's external GPU. But it's so, so not worth it."

https://mashable.com/review/blackmagic-egpu-review/?europe=true#i1ixuiGsDmqD

Hi

This is an external (high power) graphics card. You are extremely unlikely to need one to run LPX.

CCT

It just seems bassackwards.... I usually know when I have a need, instead of trying to manufacture one.

Learn well to use the least of what you have, that prepares you for the next stage, there's almost no substitute for learning the basics. The one exception I can think of is pure inspiration, add to that good instincts/intuition and you can circumvent some of the learning process. I've done both... at 13 a friend of mine got a sears electric guitar and amp... the British invasion was just beginning.. he gave up playing in a few weeks and I sold my motor bike buy the guitar/amp. My first inspiration was thousands of screaming girls. :-) .

Remember, it's Never about the gear, with a little talent and perseverance you can produce high quality music on a four track cassette recorder if you -know- what you're doing. You just have to want it, the higher you place something in your life (aspiration) the more you will work to achieve it.

Hi

BlackMagic make a variety of products. Which one are you referring to?

CCT

You don't need any Occult materials for Logic. But as a musician, it helps.

You don't know what it is and you ask if you need it? Makes absolutely no sense.