Ergates and Jesus_Phish are the two people who are mainly correct on this topic.

In terms of your credit rating (or 'blacklisting' as you put it) - this absolutely does not apply to you outside of the UK. Running up a debt, especially for as something as minor as a telephone contract, will not affect your credit rating in France. Failure to pay will result in the service being disconnected and the company will put a default on your credit file to reflect that you have not maintained payments for the duration of your contract - but no French company will be able to see this if it's done in the UK.

OP, your original idea of letting the debt be sold on to a debt purchaser is not a bad idea. However, even if you then settle the debt at a discount, you credit file will only be updated to mark the default as partially satisfied with a balance of 0.00. The default will technically still be there, but there's something on it to reflect that you've paid it off. There are circumstances in which a default can be completely removed, but do not count on it if you just can't afford to pay because of your financial situation.

Anyway, lack of shared credit files with the wider EEA means that, if you do run up a debt and move abroad, the company will be totally unable to trace you - unless you do actually give them your French address voluntarily.

TL;DR: No, running up a debt in the UK will not affect your ability to get credit in France. Unless you're a serial, serial fraudster and Interpol get involved or some of that crazy shit.