I've used a similar export in v13 via the Rest API. Not to get the signatures per signature set, but all signatures from policies in blocking.
Specifically getting the attack signature sets to correlate to the signatures assigned to the blocking policy would require some additional logic and leg work.
API endpoints:
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You can find policies in Blocking:
API endpoint: https://$target_ip/mgmt/tm/asm/policies
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Signature sets for those policies:
API endpoint: https://$target_ip/mgmt/tm/asm/policies//signature-sets
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Signatures for the policies:
API endpoint: https://$target_ip/mgmt/tm/asm/policies//signatures?\$expand=signatureReference