What became Normandy was, since the mid-IXth century, considered as Neustria. While the name existed since the late Merovingian period to name the formalized Frankish sub-kingdom, it took a more territorial and a different political meaning during the late Carolingian period.
In 861 (maybe 862?), Charles the Bald created two Marchs of Neustria to protect the region against, respectively, Bretons and Vikings attacks, Paris having been looted and burned by Normans, and Thérouanne shortly thereafter.
The march of Neustria against Bretons (for the sake of clarity, let's call it the Breton March) is set to counter the Breton advance in Neustria, after that a first Breton March and other territories were lost in the 840's and 850's, and to relocate the military command of the Duchy of Cenomans (Le Mans) in Tours in order to protect Maine and Anjou. Initially under the command of Louis, son of Charles, but after he revolted against his father, it was trusted to Robertians (the direct ancestors of Capetians) and in spite of some attempts to reassert royal command-such as in 866), they never let it go.
The march of Neustria in modern Normandy (for the aforementioned reason, let's call it the Coastal March) was made up of the territory between Seine river and Maine knew a similar fate, being trusted with royal appointees but before the hostile reaction of the local leading family, the Rorgronids (distant ancestors of the Plantagenets and seemingly closely related to Robertians), they finally obtained it. Fighting against Bretons and Vikings, they were also their immediate intermediaries with the Western Frankish kings and had no qualm allying with them if needed.
Eventually, after Rorgonid family slowly vanished, Western Frankish kings were able to appoint their own men as marchios, but it's still debated if they settled down or not. After Robertian took over the throne, we don't know if they (or later Carolingian kings) appointed another marchio in this march, giving its state, there as maybe no real point doing so.
In fact, neither march can be really considered as principalities : as it happened in the former Neustria, political power and structures were scattered and prone to infighting, and Late Carolingian Neustria began to be considered a bit distant from the rest of the kingdom (including parts of former Neustria). Hincmar of Reims, for instance, singles the region as Transsequania (the land beyond Seine) and its inhabitants being considered a bit apart from other Franks. It's true that not only Bretons, Vikings but as well Saxons lived there and Franks had no real problem dealing with them locally.
Eventually the whole region, if especially modern Normandy, a good part of the Breton March could be incuded, formed some sort of Late Carolingian "wild west" with a lack of authority (both Robertian or Carolingian) which allowed opportunities and opportunists to raise and particularily so with the extinction of Rorgonids : Maine was successfully disputed by a rogue count Roger of debated origin (Rorgonid? Semi-Rorgonid? Tied to Robertians?)
The regional command were hijacked by these families in order to both strengthen their local power but as well prevent the king or rivals to be an obstacle : they tended to be more or less superficial commands consecrating the reality of aristocratic power. The Breton march was often redrawn following new necessities but as well realities of a growing Robertian power in the build-up of the Duchy of France; and the Coastal March was more or less reduced to a disorganized mess where entire parts were left to themselves (most noticeably in western Normandy).
As Robertians take a prominent place in western Francia, they left effective control of the Breton March to other families (such as Ingelgerians, predecessor of Plantagenets) while keeping the title of marchio and when Rollo was gaven what was the former centre of the Coastal March, Robert obtained the dual margraviat as demarcus but it gave him no further power (except maybe some legitimacy to deal in the region,which he already did), and Normans simply took over the disordered regions of the former Coastal March.
In short, Normandy was one of the Marches of Neustria, and considered as such before 911, but wasn't singled as a specific region besides an administrative division. In 950's, while Normandy territorial build-up was mostly over upon the former Coastal March, it didn't prevented Thibault de Blois and Foulques d'Anjou to present themselves as governors and administrators of the Kingdom of Neustria
- L'aventure des Normands, François Neveux, 2006
- La destinée de la Neustrie du IXè au XIè, Jacques Boussard, 1968
- Les Vikings et le monde Franc, Pierre Baudin, 2009