Is it really necessary to leave a completely blank line between the CSS curly open and closing bracket?

Functionally, you can absolutely write the whole thing on a single line. Separating them out is a stylistic choice and best practice for ease of reading, more so when multiple properties are being set.

#wrapper { background: blue;}

is going to do exactly the same things as

#wrapper {
  background: blue;
}

Sure, the browser doesn't care. All of these are valid

.box { background: green; }
.box {
  background: green;
}
.box{background:green;padding:10px;margin:7px;border:4px solid black;border-raidus:2px;}
.box {
  background: green;
  padding: 10px;
  margin: 7px;
  border: 4px solid black;
  border-radius: 2px;
}

It all just comes to what's readable to you, and whatever CSS style guide you might be working from, which would be common on projects with multiple coders.

Typically, if it's a rule with only one attribute, it's put on one line, any more than one, each attribute goes on it's own line for readability sake.

What would be more common, is to see the human readable version where each property has it's own line, and than let a css parser minify the code so it removes whitespace and linebreaks. You'll learn about this later, don't worry about it today. Just know there are solutions for this.

Wow I didn't actually expect the question answered this soon. It's really helpful and thank you a lot. It's such a nice thing to have someone there for you in the online learning process.