I'd go with the following: Because what you describe is basically Mordhau. It was a real thing in medieval sword combat... but it was an improvised use of the weapon. But rather spending a feat just to do a Mordhau, I'd rather suggest to spend a downtime and a few gp to simply train that attack with a teacher for improvised weapons.
For more information about Mordhau... here's its disappointingly short wikipedia article:
In the German school of swordsmanship, Mordhau, alternatively
Mordstreich or Mordschlag (Ger., lit., "murder-stroke" or
"murder-strike" or "murder-blow"), is a half-sword technique of
holding the sword inverted, with both hands gripping the blade, and
hitting the opponent with the pommel or crossguard. This technique
allows the swordsman to essentially use the sword as a mace or hammer.
The Mordhau is mainly used in armoured combat, although it can be used
to surprise an opponent in close quarters.
and here's a video
RAW it's totally fine to use a weapon as improvised.
Sometimes characters don't have their weapons and
have to attack with whatever is close at hand. An improvised weapon
includes any object you can wield in one or two hands, such as broken
glass, a table leg, a frying pan, a wagon wheel, or a dead goblin.
In many cases, an improvised weapon is similar to an actual weapon and
can be treated as such. For example, a table leg is akin to a club. At
the DM's option, a character proficient with a weapon can use a
similar object as if it were that weapon and use his or her
An object that bears no resemblance to a weapon deals 1d4 damage (the
DM assigns a damage type appropriate to the object). If a character
uses a ranged weapon to make a melee attack, or throws a melee weapon
that does not have the thrown property, it also deals 1d4 damage. An
improvised thrown weapon has a normal range of 20 feet and a long
range of 60 feet.
So, a longsword wielded with Mordhau could resemble a Warhammer or a Greatclub. So if you're proficient with Warhammers or Greatclubs and Longswords you theoretically could use your Longsword with Mordhau.
Because an improvised weapon can be
any object you can wield in one or two hands
In many cases, an improvised weapon is similar to an actual weapon and can be treated as such
So it's pretty much up to your DM whether he or she thinks a Mordhau Longsword is a Warhammer, a great club or a two handed weapon that deals 1d4 bludgeoning damage.
TBH every character that is proficient with simple and martial weapons should be able to perform a Mordhau or a similar strike. I mean it's a very minor tweak and implements a very interesting sword technique.
For the Blade Warlock it's a little bit different anyways, because you are always proficient with your pact weapon. To switch between slashing and bludgeoning for a damage decrease is a pretty okay effect that shouldn't be take too much effort for your character. Sure it's a bit of a stretch of improvised weapons to use actual weapons as improvised weapons... but I really don't see any occasion that it could break the game.
Comment me, if I miss smth.