Possibility of blacklisted OFW - signed an offer letter from an agency

Good evening, may I ask about contract breach say for instance I signed an offer letter from an agency and there is another contract from my original agency I haven't signed yet? Could I be considered as black listed by the other agency from which I signed my signature? I said or requested to cancel their offer letter?

The point of law is that once you have signed the acceptance and communicated it, you are bound by it. You cannot unilaterally withdraw from it. If you do the legal consequences may follow. You can revoke your acceptance before your acceptance had reached the employer otherwise it would be subject to the agreement of the employer for it.
This is subject to contract law applicable in the matter

In india, the law does not permit forced labour and therefore no one can be made to work where the do not want to. However, the contract signed is valid unless it is illegal (including illegality of contract in restrain of trade). So if you signed a contract, you are required to honour it. In this case, you are liable to pay 14 days / 1 Month salary as compensation.

However, this is not an Indian contract, so you need to speak to a lawyer who knows the law of that country (UAE).

You can always request them to cancel the offer as you have not joined. Whether they will blacklist, is a matter that is best known to them (and probably to you), so may be its better that you talk to them and see what they say.