lemauve Your account must have at least 20 XLM on it to exist (this is true for all wallets) - blackwallet allows to "bypass" that limit with the merge_account operation which consists in merging the account's funds into another wallet.
Sadly exchanges do not support that op.
So in your case, you can only send Lumens to an exchange if you have more than 20 XLM and if (balance - amount) remains greater or equal to 20 XLM. This is a Stellar limitation.
Please let me know if it's not clear enough - I will edit the merge account window to clarify that for users 😄
EDIT: Text updated, hope it's more clear for users now! Thanks for reporting that!