Who came up withe the idea of nobility and when ? I mean the hole "better blood" thing and the few noble families around europe in medieval times. Thank you :)
There's not really a direct answer to this question because it wasn't a thing that a particular person or group came up with. The origins of the idea and the social class of nobility in medieval Europe are very diverse and there's a great deal of scholarly debate over when exactly they emerge in different places, what they originate from, and so on.
Roman and (therefore) early medieval law didn't, theoretically, distinguish a particular group of "nobles", it only distinguished free and unfree people. Hence you have Thegan of Trier in around 840 remarking of a man ennobled by Louis the Pious that he had been made "free, but not noble (nobilis) because that is impossible". Of course, in practice, the distinction was made all the time, but because it originated quite informally we don't have solid data on when and how exactly it emerges, and the situation is muddied a great deal by the fact that the term nobilis itself didn't have a very stable meaning during the Middle Ages -- scholars have identified all sorts of different ways in which that term was used.
To take France as a well-studied example, from around the eleventh century a distinction was typically made between "new" nobles, who were supposed to be families that had originally been of low birth and attained power at the start of the Middle Ages during the confusion of the Carolingian era, and the more select group of older nobles who traced their lineage back before the ninth century. Now the actual validity of this divide is very much contested in current scholarship, but we can nonetheless say that some of the older nobles probably derived from the wealthy Gallo-Roman elite in late antiquity, while in the case of the new nobles they were often descended from people of low rank who became attached to the Carolingian kings in some way in the ninth century. By and large, nobles were those descended from people who had served past kings in some significant capacity.
It's important to note here in connection with your "better blood" note that the "old" nobility was not necessarily more prestigious than "new"; in some cases in fact people were specifically praised for being noble on account of their talent rather than inheritance (like Thomas de Laches talking about the Counts of Anjou for instance). In fact throughout this period a preponderant tradition of Christian thinking suggested that nobility attained by virtue was preferable to nobility through bloodline. Longstanding juristic tradition held that nobility originated by public grant and not mystical origins, though the latter was certainly often claimed by the nobles themselves. It was this idea that the more powerful monarchs who emerged after the Renaissance tapped into to justify their extended claims.
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