Use TADBlobStream to read or modify the value of BLOB field. The BLOB field is a field of one of the following data types:

• ftBLOB (LONG RAW, bytea, IMAGE, etc);
• ftMemo (LONG, TEXT, etc);
• ftWideMemo / ftFmtMemo (NCLOB, NTEXT, etc);
• ftOraBLOB (BLOB);
• ftOraClob (CLOB).

To create TADBlobStream use TADDataSet.CreateBlobStream method or just create TADBlobStream object explicitly using Create constructor. 

To modify content of BLOB field, dataset must be in dsEdit / dsInsert mode and field must be ReadOnly = False.

TADBlobStream = class(TMemoryStream);

TADDataSet.CreateBlobStream

The demo shows how to load BLOB field value from external file

ADQuery1.Append;
// assign other fields
....

// load BLOB field value
oStr := TADBlobStream.Create(ADQuery1.FieldByName('PHOTO'), bmWrite);

try
  oStr.LoadFromFile('c:\aaa.jpg');
finally
  oStr.Free;
end;
ADQuery1.Post;

The demo shows how to save BLOB field value into external file

// save BLOB field value
oStr := TADBlobStream.Create(ADQuery1.FieldByName('PHOTO'), bmRead);
try
  oStr.SaveToFile('c:\aaa.jpg');
finally
  oStr.Free;
end;

See AnyDAC\Samples\Comp Layer\TADQuery\Blobs demo for details.