The best way to say it is Je mange du chocolat. Je mange le chocolat means you’re eating a specific chocolate like someone’s chocolate. I can’t recall hearing this.

Last question first:

What is the general translation of ‘I eat chocolate.’

Qu’est-ce que tu manges au petit déjeuner ? Je mange du chocolat.

Why? Let’s answer that by studying several variations.

Variations

“Je mange du chocolat.“

I eat chocolate.

Here, “du” is totally neutral: you somehow designate the chocolate in a general way.

“Je mange un peu de chocolat.“

I eat some chocolate.

Now we insist on the fact that only a bit of it is being eaten.

“Je mange le chocolat.“

I eat the chocolate. (You know which one, the one you gave me.)

Using “le” implies you are speaking of a specific chocolate.

Conclusion

The neutral form is: “Je mange du chocolat.“

Beware though of the negative form, as was asked here before: if you want to say “I never eat chocolate”, “du” becomes “de“: “Je ne mange jamais de chocolat.“

Footnote

As highlighted by jlliagre, this is not a generic rule. It works only for uncountable.

With countables, you would not use “du” but “les“: “I like men.” would translate “J’aime les hommes.“

Countables and uncountables do not use the same sentences in French. Some examples:

• “I prefer chocolate.” = “Je préfère le chocolat.“
• “I prefer men.” = “Je préfère les hommes.“
• “I prefer that chocolate.” = “Je préfère ce chocolat.“
• “I prefer that man.” = “Je préfère cet homme.“
• “I would like some chocolate.” = “Je voudrais du chocolat.“
• “I would like some man.” = “J’aimerais un homme.“
• “There is less chocolate than before.” = “Il y a moins de chocolat qu’avant.“
• “There are fewer men than before.” = “Il y a moins d’hommes qu’avant.“