A: As you should know, 
the you can test for a blinking text for
example as follows.
  uses Crt;
  begin
    TextColor (11 + 128);  (* or LightCyan + Blink *)
    TextBackground (Blue);
    writeln ('What''s the catch?');  (* An aside, note the '' pair *)
  end.
In the above, bit 7 (the 128) controls the blinking. If you have at
least an EGA, you can alter the interpretation of the highest text
color bit to denote a hi-intensity background, but then you lose the
the blinking. The following piece of code disables blinking,
enabling a hi-intensity background.
  uses Dos;
  var regs : registers;
  begin
    FillChar (regs, SizeOf(regs), 0); (* An initialization precaution *)
    regs.ah := $10;                   (* Function $10 *)
    regs.al := $03;                   (* Subfunction $03 *)
    regs.bl := $00;
    Intr ($10, regs);      (* ROM BIOS video driver interrupt *)
  end.
To enable blinking again, set regs.bl := $01; Any high-intensity
background you may have currently on the screen, will instantly
change into a blinking text a a low-intensity background.

 A2: The previous 
answer assumes at least an EGA. Otherwise ports
must be accessed. This is both advanced and dangerous 
programming,
because errors in handling posts can do real harm. Besides it is
fair to require at least an EGA in writing modern programs, at least
for non-laptops, and on the latter the colors don't really matter
for CGA and below. Let's take a look, nevertheless, how this is 
done
for a CGA. Note that this won't work an an EGA and beyond, not at
least in my tests. For detecting the video adapter you have, see 
the
DetectGraph procedure in you Turbo Pascal manual.
   First we need some basics from MEMORY.LST in Ralf Brown's
ftp://garbo.uwasa.fi/pc/programming/inter46b.zip (or whatever
version is current):
 Format of BIOS Data Segment at segment 40h:
  63h WORD Video CRT controller base address: color=03D4h, 
mono=03B4h
  65h BYTE Video current setting of mode select register 
03D8h/03B8h
From David Jurgens's 
ftp://garbo.uwasa.fi/pc/programming/helppc21.zip
we see
  3D0-3DF Color Graphics Monitor Adapter (ports 3D0-3DB are
          write only, see 6845)
  3D8 6845 Mode control register (CGA, EGA, VGA, except PCjr)
From Darryl Friesen's (friesend@jester.usask.ca) in the late
comp.lang.pascal we have, the following procedure, with my own 
added
comments (* *).
  procedure SetBlinkState (state : boolean);
  var ModeRegPort : word;
      ModeReg     : byte;
  begin
    Inline($FA); { CLI }           (* Interrupts off *)
    ModeRegPort := MemW[$0040:$0063]+4;  (* Typically $03D4+4 
= $03D8 *)
    ModeReg := Mem[$0040:$0065];   (* Typically 1001 *)
    if state then                  (* Bit 5 controls blink enable *)
      ModeReg := ModeReg or $20    (* $20 = 00100000 (base2) *)
    else
      ModeReg := ModeReg and $DF;  (* $DF = 11011111 disable *)
    Port[ModeRegPort] := ModeReg;  (* Typically $9 = 00001001 *)
    Mem[$0040:$0065] := ModeReg;   (*       or $29 = 00101001 *)
    Inline($FB) { STI }            (* Interrupts on *)
  end;