I'll be back.和I'll come back.这种用法对吗?我会回来的.用英语正确的写法是什么样的?我英盲,

这两个都可以的

两者都对,但后者有不同/较深长的的义思.
A. 1. "I will be back in five minutes."(我出去一会儿...五分钟后我会回来)
2. "Mum, I will be back home before midnight.=I will be home by midnight."我会在午夜前回家.
B. 1. She will come back half an hour later=She will be back half an hour later.相同
2. "I will come back" = 可能意味着 " 我会恢复/东山再起..."
例: (a) The actress made a comeback years after retiring.东山再起
(b) "She will come back. Her downfall/disappearance is only temporary."东山再起

I'll be back 更恰当，表示在将来的某个时刻我会回来，说话者现在可能准备走；I'll come back 相当于I'm coming back，表示我在外面，就要回来了~~~

两种语法上来说都是对的，前者的说法更顺口些，也是比较常用的。

I'll be back.和I'll come back.两种说法都行.译成汉语都是“我会回来的”.
I'll be back.是I am back.的一般将来时,强调在家的状态.
I'll come back.是I come back的一般将来时,强调回家的动作.

两种都对，都可以用。