You probably should specify which Baldwin you want to know more about. Pretty much every Baldwin of Flanders was the son of another Baldwin of Flanders, so...

"Flanders the Lion" was supposedly the battlecry of the Flemish militia during the struggle against the French Kings, during the Battle of the Golden Spurs (and a bit before and after). They were almost certainly not used as "house words" for the Flemish counts, and certainly not for any count named Baldwin, as they all lived before that struggle.

Anyway, as said by "MootMute": there were 9 Baldwins that were counts of Flanders.

Best known are probably:

-Baldwin I "Iron-Arm", who received lands around Bruges and is commonly credited with being the first true Count of Flanders.

-Baldwin IX of Flanders, but most commonly known as Baldwin I of Constantinople, was one of the leaders of the Fourth Crusade and was put on the throne of the Latin Empire.

Ok if you are still interested:

Baldwin I ( Baudouin I ) is seen as the first count of the county of Flanders. One interesting thing is: He "kidnapped" a daughter of Charles the Bald ( Ruler of West-Francia, Italy and Holy Roman Empire and the grandson of Charlemagne) because they where in love but Charles wanted a more strategic marriage for her. They ran away and got married with the help of her brother (Louis the Stammerer). The pope then reconciled Baldwin with Charles. As part of the reconciliation Baldwin was given the rule of Pagus Flandrensis a seemingly unimportent parth of his kingdom and it was being plagued by viking attacks). But Baldwin was actually a pretty good and effective ruler. He fortified some places, stopped much of the viking attacks (his son had to deal with another series of raids) and made trade boom in the region. His line of descendents were all pretty succesfull and the Region (wich became county of Flanders became one of the richest and most densly populated regions in the whole of Europe). Later he withdrew from public life and became a monk in the abbey of Sint-Bertinus. He died there and is supposedly burried there.

If you want to know more about his descendants or Flanders in general just PM me.

Source:

J. Noterdaeme, "Boudewijn I, graaf van Vlaanderen". 1953.

H. Van Daele , "Een geschiedenis van Vlaanderen". Lannoo, 2005.

EDIT: maybe I should tell you how he got the nickname "Iron Arm"

Be noted that this is a legend and may or may not be the thruth: When he was on the run with Judith (daughter of Charles the Bald) he was traveling trough the woods in the region of Brugges in Flanders. When suddenly he was attacked by a huge bear wich was terrorizing the travelers for decenia. Baldwin immediatly threw himself at the bear and a long and epic battle ensued. When the bear stood on his hindlegs to deal a massive blow, Baldwin thrust his spear wich such force that it impaled the bear to a tree. (There is still a statue of a bear on it's hindlegs in Brugges to this day)