Because of mathematica constraints I have to make use of the BigInteger class to represent values.

After a little information I must keep result (distributed by 2x BigInteger instances) into Mysql...

What's the best datatype to keep such object ?

I believed about utilizing a Blob to keep the binary format of individuals results (128 bits) ? But I must avoid unnecessary type conversions.

I would suggest utilizing a Blob and so the BigInteger(byte[] val) constructor to visit from byte array to BigInteger, and also the BigInteger#toByteArray() way of another way.

MySQL includes a BIGINT data type as proven in:

You can test to make use of the BIGINT for your instead of doing conversion to binary formats which causes it to be good deal more complicated to my estimation.