Blob b1 = Blob.valueOf('s1');
Blob b2 = Blob.valueOf('s2');
Blob b3 = Blob.valueOf('s3');
Blob combined = b1 + b2 + b3; //Error: expression must use numeric arguments.

Blob b1 = Blob.valueOf('s1');
Blob b2 = Blob.valueOf('s2');
Blob b3 = Blob.valueOf('s3');

String combinedDataAsHex = EncodingUtil.convertToHex(b1) + EncodingUtil.convertToHex(b2) + EncodingUtil.convertToHex(b3);
Blob combinedDataAsBlob = EncodingUtil.convertFromHex(combinedDataAsHex);

Blob b1 = Blob.valueOf('s1');
Blob b2 = Blob.valueOf('s2');
Blob b3 = Blob.valueOf('s3');

String combinedDataAsBase64 = EncodingUtil.base64Encode(b1) + EncodingUtil.base64Encode(b2) + EncodingUtil.base64Encode(b3);
Blob combinedDataAsBlob = EncodingUtil.base64Decode(combinedDataAsBase64);