First let's take death metal. It has a lot of subgenres of which the most prominent is melodic death metal or gothenburg metal if you wish. Some say dm is not mdm's parent but extreme metal is the parent of both (think graph theory). It is somewhat easy to tell them apart.

Now when it comes to black metal, the label melodic black metal is also used. Although, when comparing the relation between (mbm and bm) and (mdm and dm) it is way harder to tell mbm from bm than it is to tell mdm from dm. Why is that? If I am wrong and it is not then why is melodic used as a genre label more for death metal and not for black metal.

I wish melodic death would be to death as melodic black is to black. It would make the death genre a lot more interesting =p

Well, you are right. Melodic death, or melodeath is a genre. (or subgenre, whatever) and it is very different from straightforward death metal
Meanwhile melodic black metal is not a subgenre, but the word melodic acts more like an adjective to the music, like "atmospheric", or "epic".
But why is that?, you ponder... I don't think there's a why. It's just historic I guess. Many people began calling meldic death "melodic death" and then the genre just... happened. It was not set into a date like "hereby I pronounce bands sounding like this "melodic death metal bands" or something. Same thing for melodic black.

Now to tell melodic black from black metal... Well, since melodic black metal isn't a genre per-se, I'd say it's up to the listener? Some people say a black metal is atmospheric and when I go listen I think to myself "this is not atmospheric at all", and probably the same happens for melo black, so I'd say there's no correct way of telling one and other apart, since it depends too much on the listener.

However, if I was to define, I'd say melodic black is when black metal is well... more melody driven, rather than just tremolo picking and blast beating... You know, like second wave black. Darkthrone, Marduk, Infernal War. Those bands weren't melodic at all, it was all about the blast and the violence and the hatred it infused to the soul of the listener and stupid stuff like that. And then you get Mystic Forest. It's not symphonic, not experimental. It doesn't have any new instruments or anything. It's just guitar/bass/drums black metal like everything else (ok, there may be a piano here and there but mostly on interludes, don't take that in consideration) however, the aim of the music is different. It's meant to be appreciated because of the melodies played, and not because roaaar I'm so trve I'm a second wave. Therefore I'd call it melodic black. So I guess that's it. But as I said, different people will say different things when it comes to "labelling" a black metal melodic or not.

What people call melodic death metal nowadays has nothing to do with death metal whatsoever whereas melodic black metal is still black metal in its core.

I believe the term is loosely used to describe no straightforward blastbeat necro-sound black metal .Why its not a subgenre ,well, because its still black metal ...

There is also death/doom whit out friend likes ignore, and also black/doom , atmo BM, technical DM, sypho BM, avantgard BM

all those are DM and BM supgenres, unless its doom, what are doom subgenres , and today melo death is more like happy music what CoB plays , and ppl don't label that band what they should be EPM