uses
  SynTable, SynCommons, Uni, UniProvider, ODBCUniProvider, System.NetEncoding;

procedure TForm13.Button1Click(Sender: TObject);
var w: TJSONWriter;
    f: integer;
    blob: TRawByteStringStream;
    LBytes : TArray<byte>;
    LString: string;
    bfs: TBufferedFileStream;

begin
  bfs := TBufferedFileStream.Create( 'c:\temp\xxx.json', fmCreate);
  w := TJSONWriter.Create(bfs, True, False);
  try
    while not UniQuery1.Eof do begin
        for f := 0 to UniQuery1.FieldCount - 1 do begin
          with UniQuery1.Fields[f] do
            case DataType of
            ftBytes: begin
              LBytes := AsBytes;
              LString := TEncoding.UTF8.GetString(LBytes);
              w.WrBase64(pointer(LString), length(LString), true);
            end;
            ftVarBytes, ftBlob: begin
              blob := TRawByteStringStream.Create;
              try
                (UniQuery1.Fields[f] as TBlobField).SaveToStream(blob);
                w.WrBase64(pointer(blob.DataString), length(blob.DataString), True);
              finally
                blob.Free;
              end;
            end;
          end;
        UniQuery1.Next;
      end;
      w.CancelLastComma;
      w.FlushFinal;
    end;
  finally
    w.Free;
    bfs.Free;
  end;
end;

var 
  Data: variant;
  t: string;
  field: TField;
  ss: TStringStream;
  sm: TMemoryStream;

 for field in UniQuery.Fields do begin
      case field.DataType of
        ftBoolean:
          field.AsBoolean := VarToStr(Data).ToBoolean;
        ftInteger, ftSmallint, ftShortint, ftLongWord, ftWord, ftByte:
          field.AsInteger := StrToIntDef(Data, 0);
        ftFloat, ftCurrency:
          field.AsCurrency := StrToCurr(Data);
        ftBytes:
          begin
            ss := TStringStream.Create(VarToStr(Row.Value(i)));
            try
              ss.Position := 0;
              sm := TMemoryStream.Create;
              try
                TNetEncoding.Base64.Decode(ss, sm);
                SetString(t, PChar(sm.Memory), sm.Size div SizeOf(Char));
               finally
                sm.Free;
              end;
            finally
              ss.Free;
            end;
          end;

        ftBlob:
          begin
            ss := TStringStream.Create(VarToStr(Data);
            try
              ss.Position := 0;
              sm := TMemoryStream.Create;
              try
               [TNetEncoding.Base64.Decode(ss, sm);
//в какую переменную LoadFromStream(sm) ????
               finally
                sm.Free;
              end;
            finally
              ss.Free;
            end;
    end;

  LBytes: TArray<byte>;
  bytes: TBytes;
begin
//туда
  LBytes := UniQuery1.FieldByName('yyy').AsBytes;
  s1 := TEncoding.UTF8.GetString(LBytes);
  s2 := TNetEncoding.Base64.Encode(s1);


//обратно
  s3 := TNetEncoding.Base64.Decode(s2);
  //bytes := TEncoding.UTF8.GetBytes(s3);
  LBytes := TEncoding.UTF8.GetBytes(s3);

        ftBytes: begin
          LBytes := AsBytes;
          Lstring := TEncoding.UTF8.GetString(LBytes);
          W.WrBase64(pointer(LString), length(LString), true);
        end;
        ftBlob: begin
          blob := TRawByteStringStream.Create;
          try
            (Data.Fields[f] as TBlobField).SaveToStream(blob);
            W.WrBase64(pointer(blob.DataString), length(blob.DataString), true);
          finally
            blob.Free;
          end;
        end;