Belgians have a long history of brewing in many different styles, contrasting with the German's who tend to make beer around the pale lager theme.

Strictly speaking the difference between a Belgian Blonde beer, a Dubbel & a Tripel is the amount of fermentables (usually malt sugars) in the fermentation vessel.   A Dubbel will have double the fermentable sugars & a tripel have three times the fermentables.   Clearly this will lead to stronger beers both in alcohol & flavour.

There is a little more to it as usually the Blonde is rather obviously a golden colour, a Dubbel a darker redder-browner colour & a Tripel a golden clour again.   There is no rule but the odd numbered ones are pale & even numbers darker.   Truly decadent Quadruples have come about more recently with - you've guessed it - four times the fermentables & are usually a darker chestnut towards chocolate colour.

A Tripel may well use candy sugar (which is inverted sugar syrup, where the surose has been split up into its glucose & fructose base units) to strengthen the beer without adding to the toffee-caramel full flavour.