Why do these classes only compile when in the same file?

I have this directory c:\abcd it has no files in it, but has a subdirectory efgh.

C:\abcd>dir
 Volume in drive C has no label.
 Volume Serial Number is B411-D580

 Directory of C:\abcd

02/08/2014  06:57 PM    <DIR>          .
02/08/2014  06:57 PM    <DIR>          ..
02/08/2014  07:01 PM    <DIR>          efgh
               0 File(s)              0 bytes
               3 Dir(s)   1,990,201,344 bytes free

The efgh subdirectory, as you can see, has 3 files. Blah.java, Greeter.java and comb.java

Blah.java has a class, Greeter.java has a class.

comb.java happens to contain the code of Blah.java and Greeter.java, so it has two classes. It's superfluous but it's to demonstrate something. Which is a case of comb compiling while one of the individual files (Blah.java) does not.

C:\abcd>cd efgh

C:\abcd\efgh>dir
 Volume in drive C has no label.
 Volume Serial Number is B411-D580

 Directory of C:\abcd\efgh

02/08/2014  07:01 PM    <DIR>          .
02/08/2014  07:01 PM    <DIR>          ..
02/08/2014  07:00 PM               130 Blah.java
02/08/2014  07:00 PM               216 comb.java
02/08/2014  07:00 PM                99 Greeter.java
               3 File(s)            445 bytes
               2 Dir(s)   1,990,201,344 bytes free

These are the contents of the three files

C:\abcd\efgh>type Blah.java

package efgh;

class Blah {

    Greeter greeter = new Greeter();

    public static void main(String[] args) {}

}

C:\abcd\efgh>type Greeter.java

package efgh;

class Greeter {

    public static void main(String[] args) {}

}    

C:\abcd\efgh>type comb.java

package efgh;

class Blah {

    Greeter greeter = new Greeter();

    public static void main(String[] args) {}

}

class Greeter {

    public static void main(String[] args) {}

}

C:\abcd\efgh>javac comb.java

So, comb.java compiles.

I see it produced Blah.class and Greeter.class which is further proof that it compiled, but I'll now delete those .class files. As that makes my point, that javac comb.java works.

C:\abcd\efgh>dir
 Volume in drive C has no label.
 Volume Serial Number is B411-D580

 Directory of C:\abcd\efgh

02/08/2014  07:02 PM    <DIR>          .
02/08/2014  07:02 PM    <DIR>          ..
02/08/2014  07:02 PM               341 Blah.class
02/08/2014  07:00 PM               130 Blah.java
02/08/2014  07:00 PM               216 comb.java
02/08/2014  07:02 PM               261 Greeter.class
02/08/2014  07:00 PM                99 Greeter.java
               5 File(s)          1,047 bytes
               2 Dir(s)   1,990,201,344 bytes free

C:\abcd\efgh>del *.class

Now I'll compile them individually. And it doesn't work so well.. compiling Blah.java gives an error.

C:\abcd\efgh>dir
 Volume in drive C has no label.
 Volume Serial Number is B411-D580

 Directory of C:\abcd\efgh

02/08/2014  07:03 PM    <DIR>          .
02/08/2014  07:03 PM    <DIR>          ..
02/08/2014  07:00 PM               130 Blah.java
02/08/2014  07:00 PM               216 comb.java
02/08/2014  07:00 PM                99 Greeter.java
               3 File(s)            445 bytes
               2 Dir(s)   1,990,201,344 bytes free

C:\abcd\efgh>javac Greeter.java

C:\abcd\efgh>javac Blah.java
Blah.java:5: error: cannot find symbol
   Greeter greeter = new Greeter();
   ^
  symbol:   class Greeter
  location: class Blah
Blah.java:5: error: cannot find symbol
   Greeter greeter = new Greeter();
                         ^
  symbol:   class Greeter
  location: class Blah
2 errors

I know that technically, my classpath isn't set right. Because, given that I have no classpath variable set, it is defaulting to a classpath of current directory. And given the package names, the classpath should be c:\abcd. And if I set it to that then it compiles blah.java individually.

C:\abcd\efgh>javac -cp c:\abcd Blah.java

But that still doesn't explain why comb.java compiles fine with no switches:

C:\abcd\efgh>javac comb.java

C:\abcd\efgh>

I understand the package statement in comb.java applies to the Blah class and the Greeter class. I don't see why when compiling comb.java I don't need to set the classpath specially, whereas I do when compiling javac Blah.java.