I have noticed that there are a few modern rifles that use the en-bloc. Why was the en bloc used over magazines? Was the En bloc just an odd military fad?

Sorry your wording is a little confusing. En-Bloc loaded rifles have magazines, they were just not detachable. When you say modern I am going to guess WW1-2 era, Carcano, Mannlicher, Garand, Gewehr 88 and the like modern.

So in those days most military rifles had 5 rounds and some other went up to 10. There were 2 main types of internal magazines, tubular and box. Tubular was not preferred because you had to load rounds one at a time and Spitzer or pointed bullets could not be used. Box magazines had an advantage as they could be filled in one movement.

What would become the primary way to do such was with a stripper clip. A stripper clip is a strip of metal that holds rounds stacked up (usually 5). You insert the bottom round then press down on the top and the metal strip will guide them all into the magazine. You then toss or pocket the strip.

The En-Bloc was developed first and used in the Gewehr 1888 but instead of stripping the round off a strip you shoved the whole block of ammunition in held together by a clip. The reason it didn't catch on as much as the stripper is simply that the Mauser pattern rifles used Stripper Clips and were popular (the AK of the day). If you weren't using one you were probably using a licensed copy or rifle that had a lot of influence in its design from the Mauser.

Now detachable magazines came about before rifles began adopting them. Higher capacity pistols and automatic weapons needed a lot more ammunition to reload then could fit comfortably on a clip. Bolt Action Rifles did not have that problem, a five rounder could be inserted quickly. Early semi auto rifles in full service to get the detachable box treatment were the SVT-40 and Gewehr 43. They had only 10 rounds so shooters were provided with 2 magazines and a bunch of stripper clips with one magazine being a spare in case of damage.

It wasn't until higher capacity rifles like the M14, FN Fal that these detachable magazines became popular for reloading full sized rifles. This also mostly parallel to the introduction of higher capacity assault rifles. Before that they were limited mostly to pistols, sub machine guns and light machine guns.

The En-Bloc Clip is basically the lesser brother to the Stripper Clip that didn't reach popularity because the Mauser was the Rifle everyone copied. Still well over 10 million rifles used the En-Bloc system so it certainly wasn't a dud or a rarity.

I hope that answers your question.