Discord.py Commands within Commands?

I don't really know how to play Blackjack, so I'm afraid I won't be able to give you a full answer to your question. However I will say how you can achieve what you want. There are two ways you can go about doing this in my opinion.

Method 1

Waiting for the user to react to your bot's message

For this, you have to use:

reaction, user = await client.wait_for('reaction_add', timeout=60.0, check=check)

For example, say you are waiting for 🇦 or 🅱️ from the user (This can mean hit and stand respectively). The code would look something like this:

@client.command()
async def start(ctx):
    def check(reaction, user):
        return (user == ctx.author) and (str(reaction.emoji) == '🇦' or str(reaction.emoji) == '🅱️')

    async def sendMessage(msg):
        message = await ctx.send(msg)
        await message.add_reaction('🇦')
        await message.add_reaction('🅱️')

        try:
            reaction, user = await client.wait_for('reaction_add', timeout = 60.0, check = check)
        except:
            await message.clear_reactions()
            await ctx.send('No reaction received.')
        else:
            await message.clear_reactions()
            return reaction

        return 0

    reaction = str(await sendMessage('This is my message'))

This is a simple code to check if the user reacts with 🇦 or 🅱️. You'll have to add more conditions and loops to get what you desire.

Method 2

Waiting for the user to send a message

For this, you have to use:

msg = await client.wait_for('message', check = check, timeout = 60.0)

You'll have to then check if msg equals hit or stand or some short form like h or s. Also be sure to write a check(author) function that is called inside the client.wait_for() function (check = check) to check if the author is that same as the one that ran the command.

I hope you'll be able to come up with the code you are looking for after reading this answer.

discord.py has built-in subcommand support, here's an example:

@commands.group(invoke_without_subcommand=True)
async def your_command_name(ctx):
    # Do something if there's not a subcommand invoked


@your_command_name.command()
async def subcommand_name(ctx, *args):
    # Do something

# To invoke
# {prefix}your_command_name subcommand_name some arguments here

Or you can simply wait for a message

@client.command()
async def blackjack(ctx):
    # ...
    def check(message):
        """Checks if the message author is the same as the one that invoked the 
        command, and if the user chose a valid option"""
        return message.author == ctx.author and message.content.lower() in ['stand', 'hit']

    await ctx.send('Would you like to hit or stand?')

    message = await client.wait_for('message', check=check)
    await ctx.send(f"You chose to `{message.content}`")

# To invoke
# {prefix}blackjack
# Would you like to hit or stand?
# stand
# You chose to `stand`