What's for Dinner?
A study of blue marlin stomach contents done by the University of Miami in the 1950s concluded that blue marlin are opportunistic predators that feed mostly on whatever happens to be abundant in the area. The list of species ranged from squid, spiny lobster and triggerfish to all available species of mackerel and tuna. Half-digested mola mola and small billfish pop up in marlin stomachs from time to time, and I doubt a small porpoise or turtle would escape a hungry blue either. (Big blue marlin terrify porpoise schools off Hawaii. Sometimes you'll see porpoises "shower" like ballyhoo when a large marlin is nearby.)
 
Blacks also eat anything small enough for them to take into their mouths and just about every species of bottom fish from the Great Barrier Reef has turned up in their stomachs. When short on bait, I've caught blacks on snapper, grouper and several species of jacks. (Nowadays I am more likely to switch to artificial lures when my bait supply runs low.)

Bait and Lures
To get proficient at catching both blue and black marlin, you need to master using artificial lures, live bait and dead bait. Lacking skill in any one of these techniques handicaps a crew under specific sets of conditions that will almost certainly be encountered on a regular basis.
 
Marlin are not spread evenly through the vast open waters but tend to congregrate around specific bathymetric contour lines, in well-defined temperature isotherms and especially around schools of desirable bait. A good skipper always does his homework and knows the areas where the greatest concentrations gather in relation to his port of departure. If he knows of an exact spot, like a piece of structure rising from the sea floor where he can be certain to encounter marlin, using live bait is the preferred method. Small tuna make the best possible live baits for both blue and black marlin. All species of tuna work great, but skipjack tuna (also known in some locales as aku, stripey tuna, oceanic bonito or Arctic bonito) represent the ultimate live bait.
 
Any of several species of mackerel or scad make great baits as well, and in some instances, depending on the number of baits in the area and the size of both the bait and marlin being targeted, are often the bait of choice.
 
If, as is usually the case, there's no exact spot where the skipper knows he can encounter marlin, using artificial lures and trolling at high speeds allows you to search over a large area.

Using electronics, especially echo sounders and side-scanning sonar, as well as radar to find birds that indicate the presence of bait schools, also helps savvy skippers locate marlin. 
 
When fishing deep ocean waters without bottom structure and with no visible concentrations of bait, trolling lures is the superior technique. It is, by far, the easiest way for an amateur to become a successful marlin fisherman. Driving around with dozens of proven marlin lures — available pre-rigged at any good tackle shop — has led many a rank novice to the winner's circle, even in the largest and most prestigious billfish tournaments.
 
In the rare event that you find yourself in a place where either blue or black marlin are plentiful in large numbers and snapping pretty well, trolling with dead natural bait is the best tactic. When you hear of a boat catching phenomenal numbers of large marlin (say 250 pounds and up), you can bet the farm the crew used dead natural bait to catch them. Fishing dead baits requires the most skill out of both the angler and crew, but the rewards are spectacular. 
 
In a huge generalization, with many exceptions, I can state that blue marlin generally feed more aggressively than blacks. Both species frequently "crash" a trolled bait, but blacks are more likely to follow a bait for prolonged periods before striking. I've seen black marlin follow a series of dead baits for as long as 45 minutes before striking. It's rare to get a blue to follow for more than a minute or two. 
 
If one comes up in the spread, the skipper should always turn down-sea, as this will dramatically increase the length of time the marlin will follow the boat and may allow the crew to pitch a live or dead bait to the fish.
 
Pitch baiting is an excellent way to catch world records on light tackle, or entice marlin to eat a fly, but you need an expert crew on board to execute it. Even the best crews should catch more marlin trolling a mixture of hooked lures or bait than relying completely on the bait-and-switch.