The Ross, Skye and Lochaber MP said he was “devastated” by the idea of Brexit in a Commons debate on the European Union (Withdrawal) Act.
Mr Blackford urged MPs to “act to stop the greatest act of self-harm to our economy” by voting down the withdrawal agreement.
He said: “To stand here today with only 17 days to go until we leave the EU, to know that Scotland’s historic place in Europe is under threat is devastating ...
“The Prime Minister believes she can fool us the way she has fooled those on her own benches.
“We won’t be fooled. Nothing has changed ... this deal isn’t a new deal. It’s the same deal and it’s the same bad deal for Scotland.”
Mr Blackford would not be drawn on the impact voting down the deal could have on the Northern Irish border.
Independent unionist MP Lady Sylvia Hermon stressed her concerns that voting down Mrs May’s deal left Northern Ireland at risk.
She said: “Could I ask him to look at the wider country of the UK and explain to this House before we vote tonight the consequences of leaving the EU without a deal and the consequences particularly for Northern Ireland.
“Many businesses, many farmers, many fishermen, many people, many community leaders support the PM’s Brexit deal.
“I do not want violence back on the border again from dissident republicans.”
Mr Blackford said the people of Scotland “wish to have our rights as EU citizens protected”, so his party would vote against the deal, but also against a no-deal Brexit, which risked issues around the border.
He said: “I hope this House overwhelmingly rejects the Prime Minister’s deal tonight.
“But tomorrow we must take our responsibilities and vote down no deal, which is catastrophic.”
The SNP Westminster leader said he was “deeply sad” to reach a point of “complete crisis” on Brexit and said the Tory party’s internal battles were to blame.
He said: “The historic achievement of the European project unravelling and for what?
“To replace partnership and stability with isolationism and chaos.
“Let’s not beat about the bush, this battle began in the Tory Party and there it should have stayed.”